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MySQL常用函数介绍

　　　　　　　　　　　　　　　　MySQL常用函数介绍　　　　　

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　作者：尹正杰

一.操作符介绍

1>.操作符优先级　　

mysql> SELECT 10+20*30;
+----------+
| 10+20*30 |
+----------+
|      610 |
+----------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT (10+20)*30;
+------------+
| (10+20)*30 |
+------------+
|        900 |
+------------+
1 row in set (0.00 sec)

mysql> 
mysql>

如果想改变优先级执行顺序，则可以使用括号(mysql> SELECT (10+20)*30;)

2>.对比操作符

• 对比操作符的执行结果为true,false,null三种

• Between A and B 代表检查值是否在A和B之间

• Coalesce()代表返回第一个非Null的值

• =代表相等操作符

• >代表大于操作符

• >=代表大于等于操作符

• Greatest()代表返回最大的值

• In()代表检查值是否在一系列的值之中

• Interval()代表返回比第一个参数小的参数的位置

• is/is not代表检查值是否与布尔值相同/不同 • Is not null代表检查值是否是非NULL

• Is null代表检查值是否是NULL

• Isnull()代表检查参数是NULL

• Least()代表返回最小的参数 • <代表小于操作符

• <=代表小于等于操作符

• Like代表字符匹配

• Not between A and B代表检查值是否不在A和B的范围之内 • !=/<>代表不等于操作符

• Not in()代表检查值是否不在一系列值的当中 • Not like代表检查值是否不匹配

• Strcmp()对比两个字符串

mysql> SELECT 1 = 0;
+-------+
| 1 = 0 |
+-------+
|     0 |
+-------+
1 row in set (0.01 sec)

mysql> 
mysql> SELECT "0" = 0; 
+---------+
| "0" = 0 |
+---------+
|       1 |
+---------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT "0.0" = 0;
+-----------+
| "0.0" = 0 |
+-----------+
|         1 |
+-----------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT "0.0001" = 0; 
+--------------+
| "0.0001" = 0 |
+--------------+
|            0 |
+--------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT ".01" = 0.01;
+--------------+
| ".01" = 0.01 |
+--------------+
|            1 |
+--------------+
1 row in set (0.01 sec)

mysql>

“=” 对比操作符案例展示

mysql> SELECT ".01" <> "0.01";
+-----------------+
| ".01" <> "0.01" |
+-----------------+
|               1 |
+-----------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT .01 <> "0.01";  
+---------------+
| .01 <> "0.01" |
+---------------+
|             0 |
+---------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT .01 != "0.01";   
+---------------+
| .01 != "0.01" |
+---------------+
|             0 |
+---------------+
1 row in set (0.00 sec)

mysql> mysql> 
mysql> SELECT "yinzhengjie" <> "zhengjie";
+-----------------------------+
| "yinzhengjie" <> "zhengjie" |
+-----------------------------+
|                           1 |
+-----------------------------+
1 row in set (0.00 sec)

mysql> SELECT "yinzhengjie" != "zhengjie";   
+-----------------------------+
| "yinzhengjie" != "zhengjie" |
+-----------------------------+
|                           1 |
+-----------------------------+
1 row in set (0.00 sec)

mysql>

"<>/!=" 对比操作符案例展示

mysql> SELECT 2 <= 3;
+--------+
| 2 <= 3 |
+--------+
|      1 |
+--------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 3 <= 2;
+--------+
| 3 <= 2 |
+--------+
|      0 |
+--------+
1 row in set (0.00 sec)

mysql> 
mysql>

"<=" 小于等于对比操作符案例展示

mysql> SELECT 100 < 200;
+-----------+
| 100 < 200 |
+-----------+
|         1 |
+-----------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 100 < 100;
+-----------+
| 100 < 100 |
+-----------+
|         0 |
+-----------+
1 row in set (0.00 sec)

mysql> 
mysql>

"<" 小于对比操作符案例展示

mysql> SELECT 100 >= 200;
+------------+
| 100 >= 200 |
+------------+
|          0 |
+------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 200 >= 100;
+------------+
| 200 >= 100 |
+------------+
|          1 |
+------------+
1 row in set (0.00 sec)

mysql> 
mysql>

">=" 大于等于对比操作符

mysql> SELECT 200 > 100; 
+-----------+
| 200 > 100 |
+-----------+
|         1 |
+-----------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 100 > 200; 
+-----------+
| 100 > 200 |
+-----------+
|         0 |
+-----------+
1 row in set (0.00 sec)

mysql>

">" 大于对比操作符

mysql> SELECT 100 IS TRUE, 0 IS FALSE, NULL IS UNKNOWN;
+-------------+------------+-----------------+
| 100 IS TRUE | 0 IS FALSE | NULL IS UNKNOWN |
+-------------+------------+-----------------+
|           1 |          1 |               1 |
+-------------+------------+-----------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT 200 IS TRUE, 200 IS FALSE, 200 IS UNKNOWN;      
+-------------+--------------+----------------+
| 200 IS TRUE | 200 IS FALSE | 200 IS UNKNOWN |
+-------------+--------------+----------------+
|           1 |            0 |              0 |
+-------------+--------------+----------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 0 IS TRUE, 0 IS FALSE, 0 IS UNKNOWN;          
+-----------+------------+--------------+
| 0 IS TRUE | 0 IS FALSE | 0 IS UNKNOWN |
+-----------+------------+--------------+
|         0 |          1 |            0 |
+-----------+------------+--------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT NULL IS TRUE, NULL IS FALSE, NULL IS UNKNOWN;   
+--------------+---------------+-----------------+
| NULL IS TRUE | NULL IS FALSE | NULL IS UNKNOWN |
+--------------+---------------+-----------------+
|            0 |             0 |               1 |
+--------------+---------------+-----------------+
1 row in set (0.00 sec)

mysql>

"is" 操作符

mysql> SELECT 10 IS NOT UNKNOWN, 0 IS NOT UNKNOWN, NULL IS NOT UNKNOWN;
+-------------------+------------------+---------------------+
| 10 IS NOT UNKNOWN | 0 IS NOT UNKNOWN | NULL IS NOT UNKNOWN |
+-------------------+------------------+---------------------+
|                 1 |                1 |                   0 |
+-------------------+------------------+---------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 10 IS NOT TRUE,0 IS NOT FALSE,NULL IS NOT UNKNOWN;
+----------------+----------------+---------------------+
| 10 IS NOT TRUE | 0 IS NOT FALSE | NULL IS NOT UNKNOWN |
+----------------+----------------+---------------------+
|              0 |              0 |                   0 |
+----------------+----------------+---------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 10 IS NOT FALSE,0 IS NOT TRUE,NULL IS NOT UNKNOWN;
+-----------------+---------------+---------------------+
| 10 IS NOT FALSE | 0 IS NOT TRUE | NULL IS NOT UNKNOWN |
+-----------------+---------------+---------------------+
|               1 |             1 |                   0 |
+-----------------+---------------+---------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 10 IS NOT FALSE,0 IS NOT TRUE,100 IS NOT UNKNOWN;    
+-----------------+---------------+--------------------+
| 10 IS NOT FALSE | 0 IS NOT TRUE | 100 IS NOT UNKNOWN |
+-----------------+---------------+--------------------+
|               1 |             1 |                  1 |
+-----------------+---------------+--------------------+
1 row in set (0.00 sec)

mysql>

"Is not" 操作符

mysql> SELECT 1 IS NULL, 0 IS NULL, NULL IS NULL;
+-----------+-----------+--------------+
| 1 IS NULL | 0 IS NULL | NULL IS NULL |
+-----------+-----------+--------------+
|         0 |         0 |            1 |
+-----------+-----------+--------------+
1 row in set (0.00 sec)

mysql> 
mysql>

"Is null" 对比操作符

mysql> SELECT 1 IS NOT NULL, 0 IS NOT NULL, NULL IS NOT NULL;
+---------------+---------------+------------------+
| 1 IS NOT NULL | 0 IS NOT NULL | NULL IS NOT NULL |
+---------------+---------------+------------------+
|             1 |             1 |                0 |
+---------------+---------------+------------------+
1 row in set (0.00 sec)

mysql>

"Is not null" 对比操作符

mysql> SELECT 2 BETWEEN 1 AND 3, 2 BETWEEN 3 and 1;
+-------------------+-------------------+
| 2 BETWEEN 1 AND 3 | 2 BETWEEN 3 and 1 |
+-------------------+-------------------+
|                 1 |                 0 |
+-------------------+-------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 1 BETWEEN 2 AND 3;
+-------------------+
| 1 BETWEEN 2 AND 3 |
+-------------------+
|                 0 |
+-------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT'b'BETWEEN'a'AND'c';
+---------------------+
| 'b'BETWEEN'a'AND'c' |
+---------------------+
|                   1 |
+---------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 2 BETWEEN 2 AND '3';
+---------------------+
| 2 BETWEEN 2 AND '3' |
+---------------------+
|                   1 |
+---------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 2 BETWEEN 2 AND 'x-3';
+-----------------------+
| 2 BETWEEN 2 AND 'x-3' |
+-----------------------+
|                     0 |
+-----------------------+
1 row in set, 1 warning (0.00 sec)

mysql>

"Expr between min and max" 对比操作符,相当于 "min <= expr AND expr <= max","expr NOT BETWEEN min AND max" 对比操作符相当于NOT(expr BETWEEN min AND max)

mysql> 
mysql> SELECT COALESCE(NULL,200);
+--------------------+
| COALESCE(NULL,200) |
+--------------------+
|                200 |
+--------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT COALESCE(NULL,NULL,NULL);
+--------------------------+
| COALESCE(NULL,NULL,NULL) |
+--------------------------+
|                     NULL |
+--------------------------+
1 row in set (0.00 sec)

mysql> 
mysql>

"COALESCE(value,...)" 对比操作符,返回第一个非NULL的值，如果没有非null值，则返回NULL

mysql> SELECT GREATEST(200,0);
+-----------------+
| GREATEST(200,0) |
+-----------------+
|             200 |
+-----------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT GREATEST(18.0,3.1415926,5.2,1314.520);
+---------------------------------------+
| GREATEST(18.0,3.1415926,5.2,1314.520) |
+---------------------------------------+
|                          1314.5200000 |
+---------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT GREATEST('B','A','C','D','E','F','G');
+---------------------------------------+
| GREATEST('B','A','C','D','E','F','G') |
+---------------------------------------+
| G                                     |
+---------------------------------------+
1 row in set (0.00 sec)

mysql>

"GREATEST(value1,value2,...)" 返回其中最大的值

mysql> SELECT 5 IN (1,5,15,20,25);
+---------------------+
| 5 IN (1,5,15,20,25) |
+---------------------+
|                   1 |
+---------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 10 IN (1,5,15,20,25); 
+----------------------+
| 10 IN (1,5,15,20,25) |
+----------------------+
|                    0 |
+----------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 'zhengjie' IN ('yinzhengjie');
+-------------------------------+
| 'zhengjie' IN ('yinzhengjie') |
+-------------------------------+
|                             0 |
+-------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 'zhengjie' IN ('yinzhengjie','zhengjie');
+------------------------------------------+
| 'zhengjie' IN ('yinzhengjie','zhengjie') |
+------------------------------------------+
|                                        1 |
+------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT(20,30) IN ((10,20),(30,40));       
+------------------------------+
| (20,30) IN ((10,20),(30,40)) |
+------------------------------+
|                            0 |
+------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT(20,30) IN ((10,20),(30,20));
+------------------------------+
| (20,30) IN ((10,20),(30,20)) |
+------------------------------+
|                            0 |
+------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT(20,30) IN ((10,20),(20,30));
+------------------------------+
| (20,30) IN ((10,20),(20,30)) |
+------------------------------+
|                            1 |
+------------------------------+
1 row in set (0.00 sec)

mysql>

"expr IN (value,...)"对比操作符,当expr值能在values中找到，则返回1，否则返回0。"expr NOT IN (value,...)" 对比操作符与其相反

mysql> SELECT ISNULL(10*20);
+---------------+
| ISNULL(10*20) |
+---------------+
|             0 |
+---------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT ISNULL(10/0);
+--------------+
| ISNULL(10/0) |
+--------------+
|            1 |
+--------------+
1 row in set, 1 warning (0.00 sec)

mysql> 
mysql>

"Isnull(expr)" 操作符,如果expr是null，则返回1，否则返回0

mysql> 
mysql> SELECT LEAST(10,20,30);
+-----------------+
| LEAST(10,20,30) |
+-----------------+
|              10 |
+-----------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT LEAST(9.8,3.1415926,5.20);
+---------------------------+
| LEAST(9.8,3.1415926,5.20) |
+---------------------------+
|                 3.1415926 |
+---------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT LEAST('D','C','A','F');
+------------------------+
| LEAST('D','C','A','F') |
+------------------------+
| A                      |
+------------------------+
1 row in set (0.00 sec)

mysql>

LEAST(value1,value2,...),返回最小值，如果其中有值为null，则返回null

3>.逻辑操作符

　　逻辑操作符返回1（TRUE），0（FALSE），或者NULL。

mysql> SELECT NOT 100;
+---------+
| NOT 100 |
+---------+
|       0 |
+---------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT NOT 0;
+-------+
| NOT 0 |
+-------+
|     1 |
+-------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT NOT NULL;
+----------+
| NOT NULL |
+----------+
|     NULL |
+----------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT !(10+20);
+----------+
| !(10+20) |
+----------+
|        0 |
+----------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT ! 10 + 12;
+-----------+
| ! 10 + 12 |
+-----------+
|        12 |
+-----------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT ! 10 + 1;
+----------+
| ! 10 + 1 |
+----------+
|        1 |
+----------+
1 row in set (0.00 sec)

mysql>

"NOT, !" 逻辑操作符代表非操作

mysql> SELECT 1 AND 1;
+---------+
| 1 AND 1 |
+---------+
|       1 |
+---------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 1 AND 0;
+---------+
| 1 AND 0 |
+---------+
|       0 |
+---------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 1 AND NULL;
+------------+
| 1 AND NULL |
+------------+
|       NULL |
+------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 0 AND NULL; 
+------------+
| 0 AND NULL |
+------------+
|          0 |
+------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT NULL AND 0; 
+------------+
| NULL AND 0 |
+------------+
|          0 |
+------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT NULL AND 1;
+------------+
| NULL AND 1 |
+------------+
|       NULL |
+------------+
1 row in set (0.00 sec)

mysql>

" And,&& "逻辑操作符

mysql> SELECT 2 OR 3;
+--------+
| 2 OR 3 |
+--------+
|      1 |
+--------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT 2 OR 0;
+--------+
| 2 OR 0 |
+--------+
|      1 |
+--------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 0 OR 0;
+--------+
| 0 OR 0 |
+--------+
|      0 |
+--------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 0 OR NULL;
+-----------+
| 0 OR NULL |
+-----------+
|      NULL |
+-----------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 1 OR NULL; 
+-----------+
| 1 OR NULL |
+-----------+
|         1 |
+-----------+
1 row in set (0.00 sec)

mysql>

"OR, ||" 逻辑操作符

mysql> SELECT 2 XOR 3;
+---------+
| 2 XOR 3 |
+---------+
|       0 |
+---------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 2 XOR 0;
+---------+
| 2 XOR 0 |
+---------+
|       1 |
+---------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 2 XOR NULL;
+------------+
| 2 XOR NULL |
+------------+
|       NULL |
+------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 2 XOR 3 XOR 4;
+---------------+
| 2 XOR 3 XOR 4 |
+---------------+
|             1 |
+---------------+
1 row in set (0.00 sec)

mysql>

"Xor" 逻辑操作符

4>.分配操作符

　　所谓的分配操作符就是赋值操作。

mysql> SELECT @name,@age;      #MySQL使用“@”来声明一个变量
+-------+------+
| @name | @age |
+-------+------+
| NULL  | NULL |
+-------+------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT @name:='yinzhengjie',@age:=18;       #MySQL使用":="来进行赋值操作
+----------------------+----------+
| @name:='yinzhengjie' | @age:=18 |
+----------------------+----------+
| yinzhengjie          |       18 |
+----------------------+----------+
1 row in set, 2 warnings (0.00 sec)

mysql> 
mysql> SELECT @name,@age;
+-------------+------+
| @name       | @age |
+-------------+------+
| yinzhengjie |   18 |
+-------------+------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT * FROM yinzhengjie.teacher;
+----+-----------+-----------+
| id | name      | course_id |
+----+-----------+-----------+
|  1 | 谢霆锋    |        11 |
|  2 | 周杰伦    |         1 |
|  3 | 蔡依林    |        13 |
|  4 | 杨幂      |         2 |
|  5 | 胡歌      |        12 |
|  6 | 刘德华    |         3 |
|  7 | 张学友    |        10 |
|  8 | 郭德纲    |         4 |
|  9 | 林俊杰    |         9 |
+----+-----------+-----------+
9 rows in set (0.00 sec)

mysql> 
mysql> SELECT @lines:=COUNT(*) FROM yinzhengjie.teacher;
+------------------+
| @lines:=COUNT(*) |
+------------------+
|                9 |
+------------------+
1 row in set, 1 warning (0.00 sec)

mysql>

mysql> SELECT @name:='yinzhengjie',@age:=18; 　　　　　　　　　　　　　　#MySQL使用":="来进行赋值操作

mysql> CREATE TABLE students(id INT(11),name VARCHAR(30));  
Query OK, 0 rows affected (0.01 sec)

mysql> 
mysql> INSERT INTO students(id,name) VALUES(1,'jason'),(2,'jay'),(3,'lady Gaga'),(4,'G.E.M'),(5,'JJ lin');
Query OK, 5 rows affected (0.00 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> 
mysql> SELECT * FROM students;
+------+-----------+
| id   | name      |
+------+-----------+
|    1 | jason     |
|    2 | jay       |
|    3 | lady Gaga |
|    4 | G.E.M     |
|    5 | JJ lin    |
+------+-----------+
5 rows in set (0.00 sec)

mysql> 
mysql> UPDATE students SET name = 'yinzhengjie' WHERE id = @number:=1;
Query OK, 1 row affected, 1 warning (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 1

mysql> 
mysql> SELECT * FROM students;
+------+-------------+
| id   | name        |
+------+-------------+
|    1 | yinzhengjie |
|    2 | jay         |
|    3 | lady Gaga   |
|    4 | G.E.M       |
|    5 | JJ lin      |
+------+-------------+
5 rows in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT @number;
+---------+
| @number |
+---------+
|       1 |
+---------+
1 row in set (0.00 sec)

mysql

:=操作符也可以用在update等语句

　　"=" 操作符在两种情况下会被认为是赋值操作，而其他情况下会认为是对比操作符。

　　　　第一：在SET语句中，“=”操作符会被认为是赋值操作；

　　　　第二：在“update”语句中的SET子句中，“=”操作符也会被认为是赋值操作。

mysql> SELECT @QQ;
+------+
| @QQ  |
+------+
| NULL |
+------+
1 row in set (0.00 sec)

mysql> 
mysql> SET @QQ = 1053419035;
Query OK, 0 rows affected (0.00 sec)

mysql> 
mysql> SELECT @QQ;
+------------+
| @QQ        |
+------------+
| 1053419035 |
+------------+
1 row in set (0.00 sec)

mysql> 
mysql>

mysql> SET @QQ = 1053419035;

mysql> CREATE TABLE students(id INT(11),name VARCHAR(30));  
Query OK, 0 rows affected (0.01 sec)

mysql> 
mysql> INSERT INTO students(id,name) VALUES(1,'jason'),(2,'jay'),(3,'lady Gaga'),(4,'G.E.M'),(5,'JJ lin');
Query OK, 5 rows affected (0.00 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> 
mysql> SELECT * FROM students;
+------+-----------+
| id   | name      |
+------+-----------+
|    1 | jason     |
|    2 | jay       |
|    3 | lady Gaga |
|    4 | G.E.M     |
|    5 | JJ lin    |
+------+-----------+
rows in set (0.00 sec)

mysql> 
mysql> UPDATE students SET name = 'yinzhengjie' WHERE id = @number:=1;
Query OK, 1 row affected, 1 warning (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 1

mysql> 
mysql> SELECT * FROM students;
+------+-------------+
| id   | name        |
+------+-------------+
|    1 | yinzhengjie |
|    2 | jay         |
|    3 | lady Gaga   |
|    4 | G.E.M       |
|    5 | JJ lin      |
+------+-------------+
rows in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT @number;
+---------+
| @number |
+---------+
|       1 |
+---------+
row in set (0.00 sec)

mysql> UPDATE students SET name = 'yinzhengjie' WHERE id = @number:=1;

二.常见的函数介绍

1>.流程控制函数

mysql> ? CASE OPERATOR
Name: 'CASE OPERATOR'
Description:
Syntax:
CASE value WHEN [compare_value] THEN result [WHEN [compare_value] THEN
result ...] [ELSE result] END

CASE WHEN [condition] THEN result [WHEN [condition] THEN result ...]
[ELSE result] END

The first CASE syntax returns the result for the first
value=compare_value comparison that is true. The second syntax returns
the result for the first condition that is true. If no comparison or
condition is true, the result after ELSE is returned, or NULL if there
is no ELSE part.

*Note*:

The syntax of the CASE expression described here differs slightly from
that of the SQL CASE statement described in [HELP CASE statement], for
use inside stored programs. The CASE statement cannot have an ELSE NULL
clause, and it is terminated with END CASE instead of END.

The return type of a CASE expression result is the aggregated type of
all result values:

o If all types are numeric, the aggregated type is also numeric:

  o If at least one argument is double precision, the result is double
    precision.

  o Otherwise, if at least one argument is DECIMAL, the result is
    DECIMAL.

  o Otherwise, the result is an integer type (with one exception):

    o If all integer types are all signed or all unsigned, the result
      is the same sign and the precision is the highest of all
      specified integer types (that is, TINYINT, SMALLINT, MEDIUMINT,
      INT, or BIGINT).

    o If there is a combination of signed and unsigned integer types,
      the result is signed and the precision may be higher. For
      example, if the types are signed INT and unsigned INT, the result
      is signed BIGINT.

    o The exception is unsigned BIGINT combined with any signed integer
      type. The result is DECIMAL with sufficient precision and scale
      0.

o If all types are BIT, the result is BIT. Otherwise, BIT arguments are
  treated similar to BIGINT.

o If all types are YEAR, the result is YEAR. Otherwise, YEAR arguments
  are treated similar to INT.

o If all types are character string (CHAR or VARCHAR), the result is
  VARCHAR with maximum length determined by the longest character
  length of the operands.

o If all types are character or binary string, the result is VARBINARY.

o SET and ENUM are treated similar to VARCHAR; the result is VARCHAR.

o If all types are JSON, the result is JSON.

o If all types are temporal, the result is temporal:

  o If all temporal types are DATE, TIME, or TIMESTAMP, the result is
    DATE, TIME, or TIMESTAMP, respectively.

  o Otherwise, for a mix of temporal types, the result is DATETIME.

o If all types are GEOMETRY, the result is GEOMETRY.

o If any type is BLOB, the result is BLOB.

o For all other type combinations, the result is VARCHAR.

o Literal NULL operands are ignored for type aggregation.

URL: http://dev.mysql.com/doc/refman/8.0/en/control-flow-functions.html

Examples:
mysql> SELECT CASE 1 WHEN 1 THEN 'one'
    ->     WHEN 2 THEN 'two' ELSE 'more' END;
        -> 'one'
mysql> SELECT CASE WHEN 1>0 THEN 'true' ELSE 'false' END;
        -> 'true'
mysql> SELECT CASE BINARY 'B'
    ->     WHEN 'a' THEN 1 WHEN 'b' THEN 2 END;
        -> NULL

mysql>

查看帮助信息（mysql> ? CASE OPERATOR）

 CASE value WHEN [compare_value] THEN result [WHEN [compare_value] THEN result ...] [ELSE result] END

　　• 当value等于compare_value时，则返回result，否则返回else里的result， 如果没有else子句则返回null

mysql> SELECT CASE 100
    ->  WHEN 100 THEN '一百'
    ->  WHEN 200 THEN '二百'
    ->  ELSE '未知'
    -> END;
+-----------------------------------------------------------------------------+
| CASE 100
 WHEN 100 THEN '一百'
 WHEN 200 THEN '二百'
 ELSE '未知'
END       |
+-----------------------------------------------------------------------------+
| 一百                                                                        |
+-----------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT CASE 300 WHEN 100 THEN '一百' WHEN 200 THEN '二百' ELSE '未知' END;
+--------------------------------------------------------------------------+
| CASE 300 WHEN 100 THEN '一百' WHEN 200 THEN '二百' ELSE '未知' END       |
+--------------------------------------------------------------------------+
| 未知                                                                     |
+--------------------------------------------------------------------------+
1 row in set (0.01 sec)

mysql>

mysql> SELECT CASE 300 WHEN 100 THEN '一百' WHEN 200 THEN '二百' ELSE '未知' END;

CASE WHEN [condition] THEN result [WHEN [condition] THEN result ...] [ELSE result] END

    • 当第一个condition满足时，则返回result，否则返回else里的result，如果 没有else子句时则返回null

mysql> SELECT CASE WHEN 100 > 20 THEN '真的' ELSE '假的' END;
+----------------------------------------------------+
| CASE WHEN 100 > 20 THEN '真的' ELSE '假的' END     |
+----------------------------------------------------+
| 真的                                               |
+----------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT CASE WHEN 10 > 20 THEN 'TRUE' ELSE 'FALSE' END; 
+------------------------------------------------+
| CASE WHEN 10 > 20 THEN 'TRUE' ELSE 'FALSE' END |
+------------------------------------------------+
| FALSE                                          |
+------------------------------------------------+
1 row in set (0.00 sec)

mysql>

mysql> SELECT CASE WHEN 100 > 20 THEN '真的' ELSE '假的' END;

IF(expr1,expr2,expr3)

    • 当expr1为1/true时，则返回expr2，否则返回expr3

mysql> SELECT IF(10>5,"大于","小于");
+----------------------------+
| IF(10>5,"大于","小于")     |
+----------------------------+
| 大于                       |
+----------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT IF(10>5,"YES","NO");   
+---------------------+
| IF(10>5,"YES","NO") |
+---------------------+
| YES                 |
+---------------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT IF(STRCMP('yinzhengjie','zhengjie'),"YES","NO");
+-------------------------------------------------+
| IF(STRCMP('yinzhengjie','zhengjie'),"YES","NO") |
+-------------------------------------------------+
| YES                                             |
+-------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT IF(STRCMP('zhengjie','yinzhengjie'),"YES","NO");   
+-------------------------------------------------+
| IF(STRCMP('zhengjie','yinzhengjie'),"YES","NO") |
+-------------------------------------------------+
| YES                                             |
+-------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT IF(STRCMP('yinzhengjie','yinzhengjie'),"YES","NO");   
+----------------------------------------------------+
| IF(STRCMP('yinzhengjie','yinzhengjie'),"YES","NO") |
+----------------------------------------------------+
| NO                                                 |
+----------------------------------------------------+
1 row in set (0.00 sec)

mysql>

mysql> SELECT IF(10>5,"大于","小于");

IFNULL(expr1,expr2)

    • 当expr1为非null时，则返回expr1，否则返回expr2

mysql> SELECT IFNULL(10,2);
+--------------+
| IFNULL(10,2) |
+--------------+
|           10 |
+--------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT IFNULL(NULL,2);
+----------------+
| IFNULL(NULL,2) |
+----------------+
|              2 |
+----------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT IFNULL(10/0,2);
+----------------+
| IFNULL(10/0,2) |
+----------------+
|         2.0000 |
+----------------+
1 row in set, 1 warning (0.00 sec)

mysql> 
mysql> SELECT IFNULL(10/0,'NULL');
+---------------------+
| IFNULL(10/0,'NULL') |
+---------------------+
| NULL                |
+---------------------+
1 row in set, 1 warning (0.00 sec)

mysql>

mysql> SELECT IFNULL(10/0,'NULL');

NULLIF(expr1,expr2)

　　• 当expr1等于expr2时，则返回null，否则返回expr1

mysql> 
mysql> SELECT NULLIF('yinzhengjie','yinzhengjie');
+-------------------------------------+
| NULLIF('yinzhengjie','yinzhengjie') |
+-------------------------------------+
| NULL                                |
+-------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT NULLIF('yinzhengjie','zhengjie');   
+----------------------------------+
| NULLIF('yinzhengjie','zhengjie') |
+----------------------------------+
| yinzhengjie                      |
+----------------------------------+
1 row in set (0.00 sec)

mysql>

mysql> SELECT NULLIF('yinzhengjie','yinzhengjie');

2>.字符串函数

ASCII(str)返回str字符串中最左边字符的ascii码值，如果是空串则返回0，如果str是null则返回null

CHAR(N,...[USINGcharset_name])将括号中的N转化成ascii码对应的字符，返回这些字符组成的字符串，其中的null会被忽略

mysql> SELECT CHAR_LENGTH('yinzhengjie');
+----------------------------+
| CHAR_LENGTH('yinzhengjie') |
+----------------------------+
|                         11 |
+----------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT CHAR_LENGTH('MySQL');
+----------------------+
| CHAR_LENGTH('MySQL') |
+----------------------+
|                    5 |
+----------------------+
1 row in set (0.00 sec)

mysql>

CHAR_LENGTH(str) 返回字符串的字符长度

mysql> SELECT CONCAT('yin','zheng','jie');
+-----------------------------+
| CONCAT('yin','zheng','jie') |
+-----------------------------+
| yinzhengjie                 |
+-----------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT CONCAT('My','NULL','SQL');
+---------------------------+
| CONCAT('My','NULL','SQL') |
+---------------------------+
| MyNULLSQL                 |
+---------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT CONCAT('My',NULL,'SQL');  
+-------------------------+
| CONCAT('My',NULL,'SQL') |
+-------------------------+
| NULL                    |
+-------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT CONCAT(3,1415,926);
+--------------------+
| CONCAT(3,1415,926) |
+--------------------+
| 31415926           |
+--------------------+
1 row in set (0.01 sec)

mysql> 
mysql> SELECT CONCAT(3.1415926);  
+-------------------+
| CONCAT(3.1415926) |
+-------------------+
| 3.1415926         |
+-------------------+
1 row in set (0.00 sec)

mysql>

CONCAT(str1,str2,...) 返回括号里所有参数字符串连接在一起，当其中有参数为NULL时则返回 NULL

mysql> SELECT CONCAT_WS('-','First name','NULL','Last name');
+------------------------------------------------+
| CONCAT_WS('-','First name','NULL','Last name') |
+------------------------------------------------+
| First name-NULL-Last name                      |
+------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT CONCAT_WS('-','First name',NULL,'Last name');  
+----------------------------------------------+
| CONCAT_WS('-','First name',NULL,'Last name') |
+----------------------------------------------+
| First name-Last name                         |
+----------------------------------------------+
1 row in set (0.00 sec)

mysql>

CONCAT_WS(separator,str1,str2,...) 返回以第一个参数为分隔符的连接后的一个字符串，当有参数为NULL时则null 被忽略

INSERT(str,pos,len,newstr) 将str中从pos位置开始后的len个字符替换成newstr字符串

INSTR(str,substr) 返回str字符串中第一个出现substr字符串的位置

mysql> SELECT LEFT('yinzhengjie2019',3);
+---------------------------+
| LEFT('yinzhengjie2019',3) |
+---------------------------+
| yin                       |
+---------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT LEFT('yinzhengjie2019',11);
+----------------------------+
| LEFT('yinzhengjie2019',11) |
+----------------------------+
| yinzhengjie                |
+----------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT LEFT('yinzhengjie2019',15);
+----------------------------+
| LEFT('yinzhengjie2019',15) |
+----------------------------+
| yinzhengjie2019            |
+----------------------------+
1 row in set (0.00 sec)

mysql>

LEFT(str,len) 返回str字符串中从左边开始的len个长度的字符

mysql> SELECT LENGTH('yinzhengjie');
+-----------------------+
| LENGTH('yinzhengjie') |
+-----------------------+
|                    11 |
+-----------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT LENGTH('2019');
+----------------+
| LENGTH('2019') |
+----------------+
|              4 |
+----------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT LENGTH('yinzhengjie2019');
+---------------------------+
| LENGTH('yinzhengjie2019') |
+---------------------------+
|                        15 |
+---------------------------+
1 row in set (0.00 sec)

mysql> 
mysql>

LENGTH(str)返回str字符串的byte字节长度

mysql> SELECT LOAD_FILE('/yinzhengjie/backup/student.bak');
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| LOAD_FILE('/yinzhengjie/backup/student.bak')                                                                                                                                   |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| 1-"尹正杰"
2-"耿宇星"
3-"陈飞"
4-"彭兴旭"
5-"李慧鹏"
6-"孟欣"
7-"鲜惠珊"
8-"陈劲"
9-"居彭阳"
10-"李嘉韵"
11-"石闹闹"
12-"肖风"
13-"刘晓江"
                                    |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT LOAD_FILE('/yinzhengjie/backup/teacher.bak');       
+----------------------------------------------+
| LOAD_FILE('/yinzhengjie/backup/teacher.bak') |
+----------------------------------------------+
| 2周杰伦1                                     |
+----------------------------------------------+
1 row in set (0.00 sec)

mysql>

LOAD_FILE(file_name) 读取文件且返回文件内容为字符串

mysql> SELECT LOCATE('yin','yinzhengjie');
+-----------------------------+
| LOCATE('yin','yinzhengjie') |
+-----------------------------+
|                           1 |
+-----------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT LOCATE('zheng','yinzhengjie');      
+-------------------------------+
| LOCATE('zheng','yinzhengjie') |
+-------------------------------+
|                             4 |
+-------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT LOCATE('jie','yinzhengjie');     
+-----------------------------+
| LOCATE('jie','yinzhengjie') |
+-----------------------------+
|                           9 |
+-----------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT LOCATE('2019','yinzhengjie');    
+------------------------------+
| LOCATE('2019','yinzhengjie') |
+------------------------------+
|                            0 |
+------------------------------+
1 row in set (0.00 sec)

mysql>

LOCATE(substr,str) 返回str字符串中第一次出现substr字符串的位置，如果没有则返回"0"

mysql> SELECT LOCATE('e','yinzhengjie','1');
+-------------------------------+
| LOCATE('e','yinzhengjie','1') |
+-------------------------------+
|                             6 |
+-------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT LOCATE('E','yinzhengjie','1'); 
+-------------------------------+
| LOCATE('E','yinzhengjie','1') |
+-------------------------------+
|                             6 |
+-------------------------------+
1 row in set (0.00 sec)

mysql> SELECT LOCATE('e','yinzhengjie','8');
+-------------------------------+
| LOCATE('e','yinzhengjie','8') |
+-------------------------------+
|                            11 |
+-------------------------------+
1 row in set (0.00 sec)

mysql> SELECT LOCATE('E','yinzhengjie','8');
+-------------------------------+
| LOCATE('E','yinzhengjie','8') |
+-------------------------------+
|                            11 |
+-------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT LOCATE('A','yinzhengjie','8'); 
+-------------------------------+
| LOCATE('A','yinzhengjie','8') |
+-------------------------------+
|                             0 |
+-------------------------------+
1 row in set (0.00 sec)

mysql>

LOCATE(substr,str,pos) 返回str字符串中从pos位置开始第一次出现substr字符串的位置，如果没有则返回"0"

mysql> SET @name1 = 'yinzhengjie';
Query OK, 0 rows affected (0.00 sec)

mysql> 
mysql> SET @name2 = BINARY 'YinZhengJie';
Query OK, 0 rows affected (0.00 sec)

mysql> 
mysql> SELECT @name1,@name2;
+-------------+-------------+
| @name1      | @name2      |
+-------------+-------------+
| yinzhengjie | YinZhengJie |
+-------------+-------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT  LOWER(@name1),LOWER(@name2),LOWER(CONVERT(@name2 USING utf8));
+---------------+---------------+-----------------------------------+
| LOWER(@name1) | LOWER(@name2) | LOWER(CONVERT(@name2 USING utf8)) |
+---------------+---------------+-----------------------------------+
| yinzhengjie   | YinZhengJie   | yinzhengjie                       |
+---------------+---------------+-----------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> 
mysql>

LOWER(str) 返回将str字符串中所有字符变换成小写后的字符串，但对二进制文本无效

mysql> SELECT LPAD('yinzhengjie',20,'-');
+----------------------------+
| LPAD('yinzhengjie',20,'-') |
+----------------------------+
| ---------yinzhengjie       |
+----------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT LPAD('yinzhengjie',15,'-'); 
+----------------------------+
| LPAD('yinzhengjie',15,'-') |
+----------------------------+
| ----yinzhengjie            |
+----------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT LPAD('yinzhengjie',11,'-'); 
+----------------------------+
| LPAD('yinzhengjie',11,'-') |
+----------------------------+
| yinzhengjie                |
+----------------------------+
1 row in set (0.00 sec)

mysql> SELECT LPAD('yinzhengjie',3,'-'); 
+---------------------------+
| LPAD('yinzhengjie',3,'-') |
+---------------------------+
| yin                       |
+---------------------------+
1 row in set (0.00 sec)

mysql> 
mysql>

LPAD(str,len,padstr) 将str的左边补充为padstr，直到补充成len长度的字符串并返回;如果str的长度比len长，则返回str中最左边开始的len长度的字符

mysql> 
mysql> SELECT LTRIM('               yinzhengjie           ');
+------------------------------------------------+
| LTRIM('               yinzhengjie           ') |
+------------------------------------------------+
| yinzhengjie                                    |
+------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT LTRIM('2019               yinzhengjie           ');
+----------------------------------------------------+
| LTRIM('2019               yinzhengjie           ') |
+----------------------------------------------------+
| 2019               yinzhengjie                     |
+----------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql>

LTRIM(str) 将str最左边的空格去掉并返回

mysql> SELECT REPEAT(' yinzhengjie ',3);
+-----------------------------------------+
| REPEAT(' yinzhengjie ',3)               |
+-----------------------------------------+
|  yinzhengjie  yinzhengjie  yinzhengjie  |
+-----------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT REPEAT(' yinzhengjie ',-1);
+----------------------------+
| REPEAT(' yinzhengjie ',-1) |
+----------------------------+
|                            |
+----------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT REPEAT(' yinzhengjie ',0);
+---------------------------+
| REPEAT(' yinzhengjie ',0) |
+---------------------------+
|                           |
+---------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT REPEAT(' yinzhengjie ',5);
+-------------------------------------------------------------------+
| REPEAT(' yinzhengjie ',5)                                         |
+-------------------------------------------------------------------+
|  yinzhengjie  yinzhengjie  yinzhengjie  yinzhengjie  yinzhengjie  |
+-------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql>

REPEAT(str,count) 将str重复count并组合成字符串返回，如果count<1，则返回空串

mysql> SELECT REPLACE("www.yinzhengjie.org.cn",'i','I');
+-------------------------------------------+
| REPLACE("www.yinzhengjie.org.cn",'i','I') |
+-------------------------------------------+
| www.yInzhengjIe.org.cn                    |
+-------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql>

REPLACE(str,from_str,to_str) 将所有str字符串中匹配from_str子串的地方都替换成to_str子字符串

mysql> SELECT REVERSE('尹正杰');
+----------------------+
| REVERSE('尹正杰')    |
+----------------------+
| 杰正尹               |
+----------------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT REVERSE('yinzhengjie');
+------------------------+
| REVERSE('yinzhengjie') |
+------------------------+
| eijgnehzniy            |
+------------------------+
1 row in set (0.00 sec)

mysql>

REVERSE(str) 将str字符串中的字符按照倒序组合并返回

mysql> 
mysql> SELECT RIGHT('yinzhengjie2019',4);
+----------------------------+
| RIGHT('yinzhengjie2019',4) |
+----------------------------+
| 2019                       |
+----------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT RIGHT('yinzhengjie2019',7);
+----------------------------+
| RIGHT('yinzhengjie2019',7) |
+----------------------------+
| jie2019                    |
+----------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT RIGHT('yinzhengjie2019',12);
+-----------------------------+
| RIGHT('yinzhengjie2019',12) |
+-----------------------------+
| zhengjie2019                |
+-----------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT RIGHT('yinzhengjie2019',15);
+-----------------------------+
| RIGHT('yinzhengjie2019',15) |
+-----------------------------+
| yinzhengjie2019             |
+-----------------------------+
1 row in set (0.00 sec)

mysql>

RIGHT(str,len) 将str字符串中从右边开始的len个字符返回

mysql> SELECT RPAD('yinzhengjie2019',20,'-');
+--------------------------------+
| RPAD('yinzhengjie2019',20,'-') |
+--------------------------------+
| yinzhengjie2019-----           |
+--------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT RPAD('yinzhengjie2019',18,'-');  
+--------------------------------+
| RPAD('yinzhengjie2019',18,'-') |
+--------------------------------+
| yinzhengjie2019---             |
+--------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT RPAD('yinzhengjie2019',15,'-'); 
+--------------------------------+
| RPAD('yinzhengjie2019',15,'-') |
+--------------------------------+
| yinzhengjie2019                |
+--------------------------------+
1 row in set (0.00 sec)

mysql> SELECT RPAD('yinzhengjie2019',11,'-'); 
+--------------------------------+
| RPAD('yinzhengjie2019',11,'-') |
+--------------------------------+
| yinzhengjie                    |
+--------------------------------+
1 row in set (0.00 sec)

mysql>
mysql> SELECT RPAD('yinzhengjie2019',3,'-'); 
+-------------------------------+
| RPAD('yinzhengjie2019',3,'-') |
+-------------------------------+
| yin                           |
+-------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql>

RPAD(str,len,padstr) 将字符串str从右边开始补充为padstr直到整体长度为len，如果str的长度本身大于len，则返回str中len长度的字符串

mysql> SELECT RTRIM('yinzhengjie       ');
+-----------------------------+
| RTRIM('yinzhengjie       ') |
+-----------------------------+
| yinzhengjie                 |
+-----------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT RTRIM('      yinzhengjie       ');
+-----------------------------------+
| RTRIM('      yinzhengjie       ') |
+-----------------------------------+
|       yinzhengjie                 |
+-----------------------------------+
1 row in set (0.00 sec)

mysql>

RTRIM(str) 将字符串str右边的空格去掉并返回

mysql> SELECT SPACE(1);
+----------+
| SPACE(1) |
+----------+
|          |
+----------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT SPACE(10);
+------------+
| SPACE(10)  |
+------------+
|            |
+------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT SPACE(20);
+----------------------+
| SPACE(20)            |
+----------------------+
|                      |
+----------------------+
1 row in set (0.00 sec)

mysql> SELECT SPACE(30);
+--------------------------------+
| SPACE(30)                      |
+--------------------------------+
|                                |
+--------------------------------+
1 row in set (0.00 sec)

mysql>

SPACE(N) 返回N个长度的空格组成的空字符串

SUBSTR(str,pos), SUBSTR(str FROM pos), SUBSTR(str,pos,len), SUBSTR(str FROM pos FOR len)
SUBSTRING(str,pos), SUBSTRING(str FROM pos),SUBSTRING(str,pos,len), SUBSTRING(str FROM pos FOR len)

　　• 如果没有len参数，则返回从pos位置开始的str中的子字符串;如果有len参数，则从pos位置开始返回str中长度为len的子字符串;如果pos为负值，则代表pos从右边开始数

mysql> 
mysql> SELECT SUBSTR('yinzhengjie2019',5);
+-----------------------------+
| SUBSTR('yinzhengjie2019',5) |
+-----------------------------+
| hengjie2019                 |
+-----------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT SUBSTRING('yinzhengjie2019',5); 
+--------------------------------+
| SUBSTRING('yinzhengjie2019',5) |
+--------------------------------+
| hengjie2019                    |
+--------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT SUBSTR('yinzhengjie2019' FROM 5);
+----------------------------------+
| SUBSTR('yinzhengjie2019' FROM 5) |
+----------------------------------+
| hengjie2019                      |
+----------------------------------+
1 row in set (0.00 sec)

mysql>
mysql> SELECT SUBSTRING('yinzhengjie2019' FROM 5);
+-------------------------------------+
| SUBSTRING('yinzhengjie2019' FROM 5) |
+-------------------------------------+
| hengjie2019                         |
+-------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT SUBSTR('yinzhengjie2019',5,8); 
+-------------------------------+
| SUBSTR('yinzhengjie2019',5,8) |
+-------------------------------+
| hengjie2                      |
+-------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT SUBSTRING('yinzhengjie2019',5,8);
+----------------------------------+
| SUBSTRING('yinzhengjie2019',5,8) |
+----------------------------------+
| hengjie2                         |
+----------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT SUBSTRING('yinzhengjie2019',-4);
+---------------------------------+
| SUBSTRING('yinzhengjie2019',-4) |
+---------------------------------+
| 2019                            |
+---------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT SUBSTR('yinzhengjie2019',-4);   
+------------------------------+
| SUBSTR('yinzhengjie2019',-4) |
+------------------------------+
| 2019                         |
+------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT SUBSTR('yinzhengjie2019',-4,5);
+--------------------------------+
| SUBSTR('yinzhengjie2019',-4,5) |
+--------------------------------+
| 2019                           |
+--------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT SUBSTR('yinzhengjie2019',-4,2);
+--------------------------------+
| SUBSTR('yinzhengjie2019',-4,2) |
+--------------------------------+
| 20                             |
+--------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT SUBSTRING('yinzhengjie2019' FROM -12 FOR 8);
+---------------------------------------------+
| SUBSTRING('yinzhengjie2019' FROM -12 FOR 8) |
+---------------------------------------------+
| zhengjie                                    |
+---------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT SUBSTR('yinzhengjie2019' FROM -12 FOR 8);   
+------------------------------------------+
| SUBSTR('yinzhengjie2019' FROM -12 FOR 8) |
+------------------------------------------+
| zhengjie                                 |
+------------------------------------------+
1 row in set (0.00 sec)

mysql>

关于SUBSTR和SUBSTRING的案例展示

mysql> SELECT SUBSTRING_INDEX('www.yinzhengjie.org.cn','.',5);
+-------------------------------------------------+
| SUBSTRING_INDEX('www.yinzhengjie.org.cn','.',5) |
+-------------------------------------------------+
| www.yinzhengjie.org.cn                          |
+-------------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT SUBSTRING_INDEX('www.yinzhengjie.org.cn','.',3);
+-------------------------------------------------+
| SUBSTRING_INDEX('www.yinzhengjie.org.cn','.',3) |
+-------------------------------------------------+
| www.yinzhengjie.org                             |
+-------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT SUBSTRING_INDEX('www.yinzhengjie.org.cn','.',1);
+-------------------------------------------------+
| SUBSTRING_INDEX('www.yinzhengjie.org.cn','.',1) |
+-------------------------------------------------+
| www                                             |
+-------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT SUBSTRING_INDEX('www.yinzhengjie.org.cn','.',-1);
+--------------------------------------------------+
| SUBSTRING_INDEX('www.yinzhengjie.org.cn','.',-1) |
+--------------------------------------------------+
| cn                                               |
+--------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT SUBSTRING_INDEX('www.yinzhengjie.org.cn','.',-3);
+--------------------------------------------------+
| SUBSTRING_INDEX('www.yinzhengjie.org.cn','.',-3) |
+--------------------------------------------------+
| yinzhengjie.org.cn                               |
+--------------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT SUBSTRING_INDEX('www.yinzhengjie.org.cn','.',-5);
+--------------------------------------------------+
| SUBSTRING_INDEX('www.yinzhengjie.org.cn','.',-5) |
+--------------------------------------------------+
| www.yinzhengjie.org.cn                           |
+--------------------------------------------------+
1 row in set (0.00 sec)

mysql>

SUBSTRING_INDEX(str,delim,count) 当count为正数，则返回delim出现在str字符串中第count次之前的子字符串，如果是负数，则从右边开始计算

mysql> SET @name = 'yinzhengjie';
Query OK, 0 rows affected (0.00 sec)

mysql> 
mysql> SET @name2 = BINARY 'YinZhengJie';
Query OK, 0 rows affected (0.00 sec)

mysql> 
mysql> SELECT @name,@name2;
+-------------+-------------+
| @name       | @name2      |
+-------------+-------------+
| yinzhengjie | YinZhengJie |
+-------------+-------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT UPPER(@name),UPPER(@name2),UPPER(CONVERT(@name2 USING utf8));
+--------------+---------------+-----------------------------------+
| UPPER(@name) | UPPER(@name2) | UPPER(CONVERT(@name2 USING utf8)) |
+--------------+---------------+-----------------------------------+
| YINZHENGJIE  | YinZhengJie   | YINZHENGJIE                       |
+--------------+---------------+-----------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql>

UPPER(str) 返回将str字符串中所有字符转换成大写的字符串,但对二进制文本无效

3>.字符串对比函数

　　用到的关键词有LIKE，NOT LIKE，STRCMP()，通配符"%"表示匹配0个或多个字符。通配符"_"表示匹配1个字符。

mysql> SELECT 'yin_zheng_jie' LIKE 'yin%';
+-----------------------------+
| 'yin_zheng_jie' LIKE 'yin%' |
+-----------------------------+
|                           1 |
+-----------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 'yin_zheng_jie' LIKE 'yin%';
+-----------------------------+
| 'yin_zheng_jie' LIKE 'yin%' |
+-----------------------------+
|                           1 |
+-----------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 'yin_zheng_jie' LIKE 'yin\_';
+------------------------------+
| 'yin_zheng_jie' LIKE 'yin\_' |
+------------------------------+
|                            0 |
+------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 'yin_zheng_jie' LIKE 'yin\_%';
+-------------------------------+
| 'yin_zheng_jie' LIKE 'yin\_%' |
+-------------------------------+
|                             1 |
+-------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 'yin_' LIKE 'yin|_' ESCAPE '|';              #当匹配字符中有特殊字符时，可以用“”或者ESCAPE来指定特殊字符为普通字符，也就是我们常说当转义。
+--------------------------------+
| 'yin_' LIKE 'yin|_' ESCAPE '|' |
+--------------------------------+
|                              1 |
+--------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 'yin_' LIKE 'yin\_';
+---------------------+
| 'yin_' LIKE 'yin\_' |
+---------------------+
|                   1 |
+---------------------+
1 row in set (0.00 sec)

mysql>

mysql> SELECT 'yin_' LIKE 'yin|_' ESCAPE '|';　　　　　　　　　　 #当匹配字符中有特殊字符时，可以用“”或者ESCAPE来指定特殊字符为普通字符，也就是我们常说当转义。

expr NOT LIKE pat [ESCAPE 'escape_char']  是上一个字符串对比函数的反义


STRCMP(expr1,expr2) 当expr1等于expr2时等于0，当expr1小于expr2时为-1，反之为1

mysql> 
mysql> SELECT STRCMP(1,2);
+-------------+
| STRCMP(1,2) |
+-------------+
|          -1 |
+-------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT STRCMP(3,2);
+-------------+
| STRCMP(3,2) |
+-------------+
|           1 |
+-------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT STRCMP('yinzhengjie','yinzhengjie2019');
+-----------------------------------------+
| STRCMP('yinzhengjie','yinzhengjie2019') |
+-----------------------------------------+
|                                      -1 |
+-----------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT STRCMP('yinzhengjie','yinzhengjie');
+-------------------------------------+
| STRCMP('yinzhengjie','yinzhengjie') |
+-------------------------------------+
|                                   0 |
+-------------------------------------+
1 row in set (0.00 sec)

mysql>

mysql> SELECT STRCMP('yinzhengjie','yinzhengjie2019');

mysql> SET @s1 = _latin1 'x' COLLATE latin1_general_ci; 
Query OK, 0 rows affected (0.00 sec)

mysql> 
mysql> SET @s2 = _latin1 'X' COLLATE latin1_general_ci; 
Query OK, 0 rows affected (0.00 sec)

mysql> 
mysql> SET @s3 = _latin1 'x' COLLATE latin1_general_cs; 
Query OK, 0 rows affected (0.00 sec)

mysql> 
mysql> SET @s4 = _latin1 'X' COLLATE latin1_general_cs; 
Query OK, 0 rows affected (0.00 sec)

mysql> 
mysql> SELECT STRCMP(@s1, @s2), STRCMP(@s3, @s4); 
+------------------+------------------+
| STRCMP(@s1, @s2) | STRCMP(@s3, @s4) |
+------------------+------------------+
|                0 |                1 |
+------------------+------------------+
1 row in set (0.00 sec)

mysql> 
mysql>

mysql> SELECT STRCMP(@s1, @s2), STRCMP(@s3, @s4);

4>.数字函数之算数操作符

/和DIV的区别：

    /代表除法;div代表整数型除法，相除之后只取整数部分

mysql> SELECT  3/5;
+--------+
| 3/5    |
+--------+
| 0.6000 |
+--------+
1 row in set (0.00 sec)

mysql> SELECT 12345/(2-1-1);
+---------------+
| 12345/(2-1-1) |
+---------------+
|          NULL |
+---------------+
1 row in set, 1 warning (0.00 sec)

mysql> 
mysql> SELECT 5 DIV 2;
+---------+
| 5 DIV 2 |
+---------+
|       2 |
+---------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT  10/3;
+--------+
| 10/3   |
+--------+
| 3.3333 |
+--------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT  10 DIV 3;
+----------+
| 10 DIV 3 |
+----------+
|        3 |
+----------+
1 row in set (0.00 sec)

mysql>

案例展示

5>.数字函数

mysql> SELECT ABS(10);
+---------+
| ABS(10) |
+---------+
|      10 |
+---------+
1 row in set (0.01 sec)

mysql> 
mysql> SELECT ABS(-10);
+----------+
| ABS(-10) |
+----------+
|       10 |
+----------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT ABS(-200);
+-----------+
| ABS(-200) |
+-----------+
|       200 |
+-----------+
1 row in set (0.00 sec)

mysql>

ABS(X) 绝对值函数

mysql> SELECT CEILING(1.23);
+---------------+
| CEILING(1.23) |
+---------------+
|             2 |
+---------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT CEILING(0.98);
+---------------+
| CEILING(0.98) |
+---------------+
|             1 |
+---------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT CEILING(3.1415926);
+--------------------+
| CEILING(3.1415926) |
+--------------------+
|                  4 |
+--------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT CEILING(9.8);
+--------------+
| CEILING(9.8) |
+--------------+
|           10 |
+--------------+
1 row in set (0.00 sec)

mysql>

CEILING(X)/CEIL(X) 返回>=X值的最小整数

mysql> SELECT FLOOR(3.14);
+-------------+
| FLOOR(3.14) |
+-------------+
|           3 |
+-------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT FLOOR(-3.14);
+--------------+
| FLOOR(-3.14) |
+--------------+
|           -4 |
+--------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT FLOOR(9.8);
+------------+
| FLOOR(9.8) |
+------------+
|          9 |
+------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT FLOOR(-9.8);
+-------------+
| FLOOR(-9.8) |
+-------------+
|         -10 |
+-------------+
1 row in set (0.00 sec)

mysql>

FLOOR(X) 返回<=X值的最大整数

mysql> SELECT MOD(10,3);
+-----------+
| MOD(10,3) |
+-----------+
|         1 |
+-----------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT MOD(108,10);
+-------------+
| MOD(108,10) |
+-------------+
|           8 |
+-------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 268 % 8;
+---------+
| 268 % 8 |
+---------+
|       4 |
+---------+
1 row in set (0.00 sec)

mysql> SELECT 108 % 10;
+----------+
| 108 % 10 |
+----------+
|        8 |
+----------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 10 % 3;
+--------+
| 10 % 3 |
+--------+
|      1 |
+--------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 10 MOD 3;
+----------+
| 10 MOD 3 |
+----------+
|        1 |
+----------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT 108 MOD 10;
+------------+
| 108 MOD 10 |
+------------+
|          8 |
+------------+
1 row in set (0.00 sec)

mysql>

MOD(N,M), N % M, N MOD M 取余数操作，返回两者相除之后剩下的余数

mysql> SELECT FLOOR(10 + (RAND() * 10));
+---------------------------+
| FLOOR(10 + (RAND() * 10)) |
+---------------------------+
|                        10 |
+---------------------------+
1 row in set (0.00 sec)

mysql> SELECT FLOOR(10 + (RAND() * 10));
+---------------------------+
| FLOOR(10 + (RAND() * 10)) |
+---------------------------+
|                        17 |
+---------------------------+
1 row in set (0.00 sec)

mysql> SELECT FLOOR(10 + (RAND() * 10));
+---------------------------+
| FLOOR(10 + (RAND() * 10)) |
+---------------------------+
|                        13 |
+---------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT * FROM yinzhengjie.student ORDER BY rand() LIMIT 1;
+----+-----------+------------+
| id | name      | teacher_id |
+----+-----------+------------+
|  2 | 尹正杰    |          1 |
+----+-----------+------------+
1 row in set (0.01 sec)

mysql>

RAND([N]) 获取0到1之间的随机小数，比如当想获取7~12之间的随机整数是可以使用

mysql> SELECT ROUND(9.8);
+------------+
| ROUND(9.8) |
+------------+
|         10 |
+------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT ROUND(3.14);
+-------------+
| ROUND(3.14) |
+-------------+
|           3 |
+-------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT ROUND(3.1415926,5);
+--------------------+
| ROUND(3.1415926,5) |
+--------------------+
|            3.14159 |
+--------------------+
1 row in set (0.00 sec)

mysql> SELECT ROUND(3.1415926,3);
+--------------------+
| ROUND(3.1415926,3) |
+--------------------+
|              3.142 |
+--------------------+
1 row in set (0.00 sec)

mysql>

ROUND(X), ROUND(X,D) 四舍五入为D位个小数，当D参数不存在是，则默认为0

mysql> SELECT TRUNCATE(3.1415926,5);
+-----------------------+
| TRUNCATE(3.1415926,5) |
+-----------------------+
|               3.14159 |
+-----------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT TRUNCATE(3.1415926,2);
+-----------------------+
| TRUNCATE(3.1415926,2) |
+-----------------------+
|                  3.14 |
+-----------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT TRUNCATE(-3.1415926,2);
+------------------------+
| TRUNCATE(-3.1415926,2) |
+------------------------+
|                  -3.14 |
+------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT TRUNCATE(1314,-2); 
+-------------------+
| TRUNCATE(1314,-2) |
+-------------------+
|              1300 |
+-------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT TRUNCATE(1314,-3);
+-------------------+
| TRUNCATE(1314,-3) |
+-------------------+
|              1000 |
+-------------------+
1 row in set (0.00 sec)

mysql> SELECT TRUNCATE(1314,-1);
+-------------------+
| TRUNCATE(1314,-1) |
+-------------------+
|              1310 |
+-------------------+
1 row in set (0.00 sec)

mysql>

TRUNCATE(X,D) 数字X只保留D位的小数，其余均舍弃

6>.日期和时间函数

mysql> SELECT ADDDATE('2014-07-18',INTERVAL 31 DAY);
+---------------------------------------+
| ADDDATE('2014-07-18',INTERVAL 31 DAY) |
+---------------------------------------+
| 2014-08-18                            |
+---------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT ADDDATE('2014-07-18',31);
+--------------------------+
| ADDDATE('2014-07-18',31) |
+--------------------------+
| 2014-08-18               |
+--------------------------+
1 row in set (0.00 sec)

mysql>

ADDDATE(date,INTERVAL expr unit), ADDDATE(expr,days) 当expr为正数时则为增加时间，为负数时则为减少时间 Unit参数可以是任意时间单位

mysql> SELECT ADDTIME('2018-12-31 23:59:59.999999', '1 1:1:1.000002');
+---------------------------------------------------------+
| ADDTIME('2018-12-31 23:59:59.999999', '1 1:1:1.000002') |
+---------------------------------------------------------+
| 2019-01-02 01:01:01.000001                              |
+---------------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT ADDTIME('01:00:00.999999', '02:00:00.999998'); 
+-----------------------------------------------+
| ADDTIME('01:00:00.999999', '02:00:00.999998') |
+-----------------------------------------------+
| 03:00:01.999997                               |
+-----------------------------------------------+
1 row in set (0.00 sec)

mysql>

ADDTIME(expr1,expr2) 将expr2的时间增加到expr1上

mysql> SELECT CONVERT_TZ('2004-01-01 12:00:00','+00:00','+10:00');
+-----------------------------------------------------+
| CONVERT_TZ('2004-01-01 12:00:00','+00:00','+10:00') |
+-----------------------------------------------------+
| 2004-01-01 22:00:00                                 |
+-----------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql>

CONVERT_TZ(dt,from_tz,to_tz) 将时间dt从from_tz这个时区转换成to_tz这个时区并返回

mysql> SELECT CURDATE();
+------------+
| CURDATE()  |
+------------+
| 2019-01-27 |
+------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT CURDATE() + 0;
+---------------+
| CURDATE() + 0 |
+---------------+
|      20190127 |
+---------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT CURDATE() + 3;
+---------------+
| CURDATE() + 3 |
+---------------+
|      20190130 |
+---------------+
1 row in set (0.00 sec)

mysql>

CURDATE()， CURRENT_DATE, CURRENT_DATE() 返回以yyyy-mm-dd或者yyyymmdd格式的当前时间

mysql> SELECT CURTIME();
+-----------+
| CURTIME() |
+-----------+
| 23:50:57  |
+-----------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT CURTIME() + 0;
+---------------+
| CURTIME() + 0 |
+---------------+
|        235111 |
+---------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT CURTIME() + 3;
+---------------+
| CURTIME() + 3 |
+---------------+
|        235117 |
+---------------+
1 row in set (0.00 sec)

mysql>

CURTIME(),CURRENT_TIME,CURRENT_TIME() 按照hh:mm:ss或者hhmmss格式返回当前时间

mysql> SELECT NOW();
+---------------------+
| NOW()               |
+---------------------+
| 2019-01-27 23:52:56 |
+---------------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT NOW() + 0;
+----------------+
| NOW() + 0      |
+----------------+
| 20190127235302 |
+----------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT NOW() + 3;
+----------------+
| NOW() + 3      |
+----------------+
| 20190127235308 |
+----------------+
1 row in set (0.00 sec)

mysql>

Now(), CURRENT_TIMESTAMP,CURRENT_TIMESTAMP() 返回当前的日期和时间，格式为yyyy-mm-dd hh:mi:ss或者 yyyymmddhhmiss

mysql> SELECT DATE('2018-12-31 01:02:03');
+-----------------------------+
| DATE('2018-12-31 01:02:03') |
+-----------------------------+
| 2018-12-31                  |
+-----------------------------+
1 row in set (0.00 sec)

mysql> 
mysql>

DATE(expr) 获取expr中的日期

mysql> SELECT DATEDIFF('2007-12-31 23:59:59','2007-12-30');
+----------------------------------------------+
| DATEDIFF('2007-12-31 23:59:59','2007-12-30') |
+----------------------------------------------+
|                                            1 |
+----------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT DATEDIFF('2010-11-30 23:59:59','2010-12-31');
+----------------------------------------------+
| DATEDIFF('2010-11-30 23:59:59','2010-12-31') |
+----------------------------------------------+
|                                          -31 |
+----------------------------------------------+
1 row in set (0.00 sec)

mysql>

DATEDIFF(expr1,expr2) 返回expr1和expr2之间的天数差异，忽略时分秒

mysql> SELECT DATE_ADD('2000-12-31 23:59:59',INTERVAL 1 SECOND);
+---------------------------------------------------+
| DATE_ADD('2000-12-31 23:59:59',INTERVAL 1 SECOND) |
+---------------------------------------------------+
| 2001-01-01 00:00:00                               |
+---------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT DATE_ADD('2010-12-31 23:59:59',INTERVAL 1 DAY);
+------------------------------------------------+
| DATE_ADD('2010-12-31 23:59:59',INTERVAL 1 DAY) |
+------------------------------------------------+
| 2011-01-01 23:59:59                            |
+------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT DATE_ADD('2100-12-31 23:59:59',INTERVAL '1:1' MINUTE_SECOND);
+--------------------------------------------------------------+
| DATE_ADD('2100-12-31 23:59:59',INTERVAL '1:1' MINUTE_SECOND) |
+--------------------------------------------------------------+
| 2101-01-01 00:01:00                                          |
+--------------------------------------------------------------+
1 row in set (0.01 sec)

mysql> SELECT DATE_SUB('2005-01-01 00:00:00',INTERVAL '1 1:1:1' DAY_SECOND);
+---------------------------------------------------------------+
| DATE_SUB('2005-01-01 00:00:00',INTERVAL '1 1:1:1' DAY_SECOND) |
+---------------------------------------------------------------+
| 2004-12-30 22:58:59                                           |
+---------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT DATE_ADD('1900-01-01 00:00:00',INTERVAL '-1 10' DAY_HOUR);
+-----------------------------------------------------------+
| DATE_ADD('1900-01-01 00:00:00',INTERVAL '-1 10' DAY_HOUR) |
+-----------------------------------------------------------+
| 1899-12-30 14:00:00                                       |
+-----------------------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT DATE_SUB('1998-01-02', INTERVAL 31 DAY);
+-----------------------------------------+
| DATE_SUB('1998-01-02', INTERVAL 31 DAY) |
+-----------------------------------------+
| 1997-12-02                              |
+-----------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT DATE_ADD('1992-12-31 23:59:59.000002',INTERVAL '1.999999' SECOND_MICROSECOND);
+-------------------------------------------------------------------------------+
| DATE_ADD('1992-12-31 23:59:59.000002',INTERVAL '1.999999' SECOND_MICROSECOND) |
+-------------------------------------------------------------------------------+
| 1993-01-01 00:00:01.000001                                                    |
+-------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT DATE_ADD('2009-01-30', INTERVAL 1 MONTH);
+------------------------------------------+
| DATE_ADD('2009-01-30', INTERVAL 1 MONTH) |
+------------------------------------------+
| 2009-02-28                               |
+------------------------------------------+
1 row in set (0.00 sec)

mysql>

DATE_ADD(date,INTERVAL expr unit), DATE_SUB(date,INTERVAL expr unit) 增加或者减少时间

mysql> SELECT DATE_FORMAT('20190127235308','%Y-%m-%D-%H-%i-%s');
+---------------------------------------------------+
| DATE_FORMAT('20190127235308','%Y-%m-%D-%H-%i-%s') |
+---------------------------------------------------+
| 2019-01-27th-23-53-08                             |
+---------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT DATE_FORMAT('20190127235308','%Y-%M-%D-%H-%m-%s'); 
+---------------------------------------------------+
| DATE_FORMAT('20190127235308','%Y-%M-%D-%H-%m-%s') |
+---------------------------------------------------+
| 2019-January-27th-23-01-08                        |
+---------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT DATE_FORMAT('2009-10-04 22:23:00', '%W %M %Y');
+------------------------------------------------+
| DATE_FORMAT('2009-10-04 22:23:00', '%W %M %Y') |
+------------------------------------------------+
| Sunday October 2009                            |
+------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT DATE_FORMAT('2009-10-04 22:23:00', '%W %M %Y');
+------------------------------------------------+
| DATE_FORMAT('2009-10-04 22:23:00', '%W %M %Y') |
+------------------------------------------------+
| Sunday October 2009                            |
+------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT DATE_FORMAT('2007-10-04 22:23:00', '%H:%i:%s');
+------------------------------------------------+
| DATE_FORMAT('2007-10-04 22:23:00', '%H:%i:%s') |
+------------------------------------------------+
| 22:23:00                                       |
+------------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT DATE_FORMAT('1900-10-04 22:23:00','%D %y %a %d %m %b %j');
+-----------------------------------------------------------+
| DATE_FORMAT('1900-10-04 22:23:00','%D %y %a %d %m %b %j') |
+-----------------------------------------------------------+
| 4th 00 Thu 04 10 Oct 277                                  |
+-----------------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT DATE_FORMAT('1997-10-04 22:23:00','%H %k %I %r %T %S %w');
+-----------------------------------------------------------+
| DATE_FORMAT('1997-10-04 22:23:00','%H %k %I %r %T %S %w') |
+-----------------------------------------------------------+
| 22 22 10 10:23:00 PM 22:23:00 00 6                        |
+-----------------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT DATE_FORMAT('1999-01-01', '%X %V');
+------------------------------------+
| DATE_FORMAT('1999-01-01', '%X %V') |
+------------------------------------+
| 1998 52                            |
+------------------------------------+
1 row in set (0.01 sec)

mysql>  SELECT DATE_FORMAT('2006-06-00', '%d');
+---------------------------------+
| DATE_FORMAT('2006-06-00', '%d') |
+---------------------------------+
| 00                              |
+---------------------------------+
1 row in set (0.00 sec)

mysql>

DATE_FORMAT(date,format) 将date日期时间转换成format格式

mysql> SELECT DAYOFMONTH('2019-01-30');
+--------------------------+
| DAYOFMONTH('2019-01-30') |
+--------------------------+
|                       30 |
+--------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT DAYOFMONTH('2007-02-03');
+--------------------------+
| DAYOFMONTH('2007-02-03') |
+--------------------------+
|                        3 |
+--------------------------+
1 row in set (0.00 sec)

mysql> 
mysql>

DAY(date), DAYOFMONTH(date) 返回date中日期在当前月份中是第几天

mysql> SELECT DAYNAME('2007-02-03');
+-----------------------+
| DAYNAME('2007-02-03') |
+-----------------------+
| Saturday              |
+-----------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT DAYNAME('2019-01-30');
+-----------------------+
| DAYNAME('2019-01-30') |
+-----------------------+
| Wednesday             |
+-----------------------+
1 row in set (0.00 sec)

mysql>

DAYNAME(date) 返回date时间是星期几

mysql> 
mysql> SELECT DAYOFWEEK('2019-10-31');
+-------------------------+
| DAYOFWEEK('2019-10-31') |
+-------------------------+
|                       5 |
+-------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT DAYOFWEEK('2019-01-30');
+-------------------------+
| DAYOFWEEK('2019-01-30') |
+-------------------------+
|                       4 |
+-------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT DAYOFWEEK('2007-02-03');
+-------------------------+
| DAYOFWEEK('2007-02-03') |
+-------------------------+
|                       7 |
+-------------------------+
1 row in set (0.00 sec)

mysql>

DAYOFWEEK(date) 返回date时间是星期“几”

mysql> SELECT DAYOFYEAR('2007-02-03');
+-------------------------+
| DAYOFYEAR('2007-02-03') |
+-------------------------+
|                      34 |
+-------------------------+
1 row in set (0.00 sec)

mysql>

DAYOFYEAR(date) 返回date是一年中的第几天，取值范围在1~366

mysql> SELECT EXTRACT(YEAR FROM '2009-07-02');
+---------------------------------+
| EXTRACT(YEAR FROM '2009-07-02') |
+---------------------------------+
|                            2009 |
+---------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> 
mysql> SELECT EXTRACT(YEAR FROM '2009-07-02');
+---------------------------------+
| EXTRACT(YEAR FROM '2009-07-02') |
+---------------------------------+
|                            2009 |
+---------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT EXTRACT(YEAR_MONTH FROM '2009-07-02 01:02:03');
+------------------------------------------------+
| EXTRACT(YEAR_MONTH FROM '2009-07-02 01:02:03') |
+------------------------------------------------+
|                                         200907 |
+------------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT EXTRACT(DAY_MINUTE FROM '2009-07-02 01:02:03');
+------------------------------------------------+
| EXTRACT(DAY_MINUTE FROM '2009-07-02 01:02:03') |
+------------------------------------------------+
|                                          20102 |
+------------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT EXTRACT(MICROSECOND FROM '2003-01-02 10:30:00.000123');
+--------------------------------------------------------+
| EXTRACT(MICROSECOND FROM '2003-01-02 10:30:00.000123') |
+--------------------------------------------------------+
|                                                    123 |
+--------------------------------------------------------+
1 row in set (0.00 sec)

mysql>

EXTRACT(unit FROM date) Unit单元和date_add/date_sub函数中的一样，是获取date日期的unit部分

mysql> SELECT FROM_UNIXTIME(1447430881);
+---------------------------+
| FROM_UNIXTIME(1447430881) |
+---------------------------+
| 2015-11-13 11:08:01       |
+---------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT FROM_UNIXTIME(1447430881) + 0;
+-------------------------------+
| FROM_UNIXTIME(1447430881) + 0 |
+-------------------------------+
|                20151113110801 |
+-------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT FROM_UNIXTIME(UNIX_TIMESTAMP(),'%Y %D %M %h:%i:%s %x');
+--------------------------------------------------------+
| FROM_UNIXTIME(UNIX_TIMESTAMP(),'%Y %D %M %h:%i:%s %x') |
+--------------------------------------------------------+
| 2019 28th January 12:12:06 2019                        |
+--------------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql>

FROM_UNIXTIME(unix_timestamp), FROM_UNIXTIME(unix_timestamp,format) 根据给定的unixtime，返回yyyy-mm-ddhh:mi:ss或者yyyymmddhhmiss格式的具体时间，或者按照format返回时间

mysql> SELECT LAST_DAY('2004-02-05');
+------------------------+
| LAST_DAY('2004-02-05') |
+------------------------+
| 2004-02-29             |
+------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT LAST_DAY('2004-03-05'); 
+------------------------+
| LAST_DAY('2004-03-05') |
+------------------------+
| 2004-03-31             |
+------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT LAST_DAY('2004-03-45');
+------------------------+
| LAST_DAY('2004-03-45') |
+------------------------+
| NULL                   |
+------------------------+
1 row in set, 1 warning (0.00 sec)

mysql>

LAST_DAY(date) 返回date日期所在月份的最后一天日期

mysql> SELECT NOW(), SLEEP(2), NOW();
+---------------------+----------+---------------------+
| NOW()               | SLEEP(2) | NOW()               |
+---------------------+----------+---------------------+
| 2019-01-28 00:14:36 |        0 | 2019-01-28 00:14:36 |
+---------------------+----------+---------------------+
1 row in set (2.00 sec)

mysql> 
mysql> SELECT SYSDATE(), SLEEP(2), SYSDATE();
+---------------------+----------+---------------------+
| SYSDATE()           | SLEEP(2) | SYSDATE()           |
+---------------------+----------+---------------------+
| 2019-01-28 00:14:46 |        0 | 2019-01-28 00:14:48 |
+---------------------+----------+---------------------+
1 row in set (2.00 sec)

mysql>

SYSDATE() 返回当前日期和时间，格式为yyyy-mm-dd hh:mi:ss或者yyyymmddhhmiss 和now()函数的区别在于now()返回的时间是语句执行的时间，而sysdate()返回的时间是该函数执行的时间

mysql> SELECT TIME('2003-12-31 01:02:03');
+-----------------------------+
| TIME('2003-12-31 01:02:03') |
+-----------------------------+
| 01:02:03                    |
+-----------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> SELECT TIME('2003-12-31 01:02:03.000123');
+------------------------------------+
| TIME('2003-12-31 01:02:03.000123') |
+------------------------------------+
| 01:02:03.000123                    |
+------------------------------------+
1 row in set (0.00 sec)

mysql>

TIME(expr) 返回expr日期时间中的时间部分

mysql> SELECT UNIX_TIMESTAMP();
+------------------+
| UNIX_TIMESTAMP() |
+------------------+
|       1548652588 |
+------------------+
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> 
mysql> SELECT UNIX_TIMESTAMP('2015-11-13 10:20:19');
+---------------------------------------+
| UNIX_TIMESTAMP('2015-11-13 10:20:19') |
+---------------------------------------+
|                            1447428019 |
+---------------------------------------+
1 row in set (0.00 sec)

mysql>

UNIX_TIMESTAMP(), UNIX_TIMESTAMP(date) 如果没有date参数，则返回当前时间到1970-01-01 00:00:00之间的秒数，如果有date参数，则表示date到1970-01-01 00:00:00之间的秒数

7>.格式转换函数

Cast()和convert()两个函数都可以用来转换数据类型或者转换字符集允许转换的数据类型包括: 
　　• Binary[N]
    • char[N]
    • Date
    • Datetime
    • decimal[M,[D]]
    • Time
    • Signed [integer]
    • Unsigned [integer]

mysql> SELECT CONVERT(_latin1'Müller' USING utf8);
+---------------------------------------+
| CONVERT(_latin1'Müller' USING utf8)  |
+---------------------------------------+
| MuÌˆller                              |
+---------------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql>

mysql> SELECT CONVERT(_latin1'Müller' USING utf8);

mysql> SELECT CONVERT('test', CHAR CHARACTER SET utf8);
+------------------------------------------+
| CONVERT('test', CHAR CHARACTER SET utf8) |
+------------------------------------------+
| test                                     |
+------------------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql>

mysql> SELECT CONVERT('test', CHAR CHARACTER SET utf8);

mysql> SELECT CAST('test' AS CHAR CHARACTER SET utf8);
+-----------------------------------------+
| CAST('test' AS CHAR CHARACTER SET utf8) |
+-----------------------------------------+
| test                                    |
+-----------------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql>

mysql> SELECT CAST('test' AS CHAR CHARACTER SET utf8);

mysql> SELECT CAST('2019-01-01' AS DATE);
+----------------------------+
| CAST('2019-01-01' AS DATE) |
+----------------------------+
| 2019-01-01                 |
+----------------------------+
1 row in set (0.00 sec)

mysql> 
mysql>

mysql> SELECT CAST('2019-01-01' AS DATE);

mysql> SELECT CONVERT('2019-01-01', DATE);
+-----------------------------+
| CONVERT('2019-01-01', DATE) |
+-----------------------------+
| 2019-01-01                  |
+-----------------------------+
1 row in set (0.01 sec)

mysql>

mysql> SELECT CONVERT('2019-01-01', DATE);

8>.聚合函数

　　聚合函数通常用在存在GROUP BY子句的语句中。

mysql> USE yinzhengjie;
Database changed
mysql> 
mysql> CREATE TABLE score_graph(
    -> id INT(11) PRIMARY KEY AUTO_INCREMENT,
    -> name VARCHAR(30),
    -> subject VARCHAR(30),
    -> score INT(3)
    -> );
Query OK, 0 rows affected (0.02 sec)

mysql> 
mysql> 
mysql> INSERT INTO score_graph(name,subject,score) VALUES('尹正杰','英语',120);
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO score_graph(name,subject,score) VALUES('尹正杰','语文',118);
Query OK, 1 row affected (0.00 sec)

mysql> 
mysql> INSERT INTO score_graph(name,subject,score) VALUES('尹正杰','数学',120);
Query OK, 1 row affected (0.01 sec)

mysql> 
mysql> INSERT INTO score_graph(name,subject,score) VALUES('尹正杰','物理',110);
Query OK, 1 row affected (0.01 sec)

mysql> INSERT INTO score_graph(name,subject,score) VALUES('尹正杰','化学',112);
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO score_graph(name,subject,score) VALUES('尹正杰','生物',108);
Query OK, 1 row affected (0.00 sec)

mysql> 
mysql> INSERT INTO score_graph(name,subject,score) VALUES('jason','语文',120);      
Query OK, 1 row affected (0.01 sec)

mysql> INSERT INTO score_graph(name,subject,score) VALUES('jason','英语',110);      
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO score_graph(name,subject,score) VALUES('jason','数学',115);      
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO score_graph(name,subject,score) VALUES('jason','物理',105);      
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO score_graph(name,subject,score) VALUES('jason','化学',102);      
Query OK, 1 row affected (0.00 sec)

mysql> 
mysql> INSERT INTO score_graph(name,subject,score) VALUES('jason','生物',98);      
Query OK, 1 row affected (0.01 sec)

mysql> 
mysql> INSERT INTO score_graph(name,subject,score) VALUES('耿宇星','语文',116);       
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO score_graph(name,subject,score) VALUES('耿宇星','数学',112);     
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO score_graph(name,subject,score) VALUES('耿宇星','英语',101);     
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO score_graph(name,subject,score) VALUES('耿宇星','物理',103);     
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO score_graph(name,subject,score) VALUES('耿宇星','化学',112);     
Query OK, 1 row affected (0.01 sec)

mysql> INSERT INTO score_graph(name,subject,score) VALUES('耿宇星','生物',78);     
Query OK, 1 row affected (0.00 sec)

mysql> 
mysql> SELECT * FROM score_graph;
+----+-----------+---------+-------+
| id | name      | subject | score |
+----+-----------+---------+-------+
|  1 | 尹正杰    | 英语    |   120 |
|  2 | 尹正杰    | 语文    |   118 |
|  3 | 尹正杰    | 数学    |   120 |
|  4 | 尹正杰    | 物理    |   110 |
|  5 | 尹正杰    | 化学    |   112 |
|  6 | 尹正杰    | 生物    |   108 |
|  7 | jason     | 语文    |   120 |
|  8 | jason     | 英语    |   110 |
|  9 | jason     | 数学    |   115 |
| 10 | jason     | 物理    |   105 |
| 11 | jason     | 化学    |   102 |
| 12 | jason     | 生物    |    98 |
| 13 | 耿宇星    | 语文    |   116 |
| 14 | 耿宇星    | 数学    |   112 |
| 15 | 耿宇星    | 英语    |   101 |
| 16 | 耿宇星    | 物理    |   103 |
| 17 | 耿宇星    | 化学    |   112 |
| 18 | 耿宇星    | 生物    |    78 |
+----+-----------+---------+-------+
18 rows in set (0.00 sec)

mysql>

创建测试数据（mysql> CREATE TABLE score_graph( id INT(11) PRIMARY KEY AUTO_INCREMENT, name VARCHAR(30), subject VARCHAR(30), score INT(3) );）

mysql> SELECT * FROM score_graph;
+----+-----------+---------+-------+
| id | name      | subject | score |
+----+-----------+---------+-------+
|  1 | 尹正杰    | 英语    |   120 |
|  2 | 尹正杰    | 语文    |   118 |
|  3 | 尹正杰    | 数学    |   120 |
|  4 | 尹正杰    | 物理    |   110 |
|  5 | 尹正杰    | 化学    |   112 |
|  6 | 尹正杰    | 生物    |   108 |
|  7 | jason     | 语文    |   120 |
|  8 | jason     | 英语    |   110 |
|  9 | jason     | 数学    |   115 |
| 10 | jason     | 物理    |   105 |
| 11 | jason     | 化学    |   102 |
| 12 | jason     | 生物    |    98 |
| 13 | 耿宇星    | 语文    |   116 |
| 14 | 耿宇星    | 数学    |   112 |
| 15 | 耿宇星    | 英语    |   101 |
| 16 | 耿宇星    | 物理    |   103 |
| 17 | 耿宇星    | 化学    |   112 |
| 18 | 耿宇星    | 生物    |    78 |
+----+-----------+---------+-------+
18 rows in set (0.01 sec)

mysql> 
mysql> SELECT name,AVG(score) FROM score_graph GROUP BY name;
+-----------+------------+
| name      | AVG(score) |
+-----------+------------+
| 尹正杰    |   114.6667 |
| jason     |   108.3333 |
| 耿宇星    |   103.6667 |
+-----------+------------+
3 rows in set (0.00 sec)

mysql> 
mysql>

AVG([DISTINCT] expr) 计算expr的平均值，distinct关键词表示是否排除重复值

mysql> SELECT * FROM score_graph;
+----+-----------+---------+-------+
| id | name      | subject | score |
+----+-----------+---------+-------+
|  1 | 尹正杰    | 英语    |   120 |
|  2 | 尹正杰    | 语文    |   118 |
|  3 | 尹正杰    | 数学    |   120 |
|  4 | 尹正杰    | 物理    |   110 |
|  5 | 尹正杰    | 化学    |   112 |
|  6 | 尹正杰    | 生物    |   108 |
|  7 | jason     | 语文    |   120 |
|  8 | jason     | 英语    |   110 |
|  9 | jason     | 数学    |   115 |
| 10 | jason     | 物理    |   105 |
| 11 | jason     | 化学    |   102 |
| 12 | jason     | 生物    |    98 |
| 13 | 耿宇星    | 语文    |   116 |
| 14 | 耿宇星    | 数学    |   112 |
| 15 | 耿宇星    | 英语    |   101 |
| 16 | 耿宇星    | 物理    |   103 |
| 17 | 耿宇星    | 化学    |   112 |
| 18 | 耿宇星    | 生物    |    78 |
+----+-----------+---------+-------+
18 rows in set (0.00 sec)

mysql> 
mysql> SELECT name,COUNT(*) FROM score_graph GROUP BY name;          
+-----------+----------+
| name      | COUNT(*) |
+-----------+----------+
| 尹正杰    |        6 |
| jason     |        6 |
| 耿宇星    |        6 |
+-----------+----------+
3 rows in set (0.00 sec)

mysql> 
mysql>

COUNT(expr) 计算expr中的个数，如果没有匹配则返回0，注意NULL的区别

mysql> SELECT * FROM score_graph;                   
+----+-----------+---------+-------+
| id | name      | subject | score |
+----+-----------+---------+-------+
|  1 | 尹正杰    | 英语    |   120 |
|  2 | 尹正杰    | 语文    |   118 |
|  3 | 尹正杰    | 数学    |   120 |
|  4 | 尹正杰    | 物理    |   110 |
|  5 | 尹正杰    | 化学    |   112 |
|  6 | 尹正杰    | 生物    |   108 |
|  7 | jason     | 语文    |   120 |
|  8 | jason     | 英语    |   110 |
|  9 | jason     | 数学    |   115 |
| 10 | jason     | 物理    |   105 |
| 11 | jason     | 化学    |   102 |
| 12 | jason     | 生物    |    98 |
| 13 | 耿宇星    | 语文    |   116 |
| 14 | 耿宇星    | 数学    |   112 |
| 15 | 耿宇星    | 英语    |   101 |
| 16 | 耿宇星    | 物理    |   103 |
| 17 | 耿宇星    | 化学    |   112 |
| 18 | 耿宇星    | 生物    |    78 |
+----+-----------+---------+-------+
18 rows in set (0.00 sec)

mysql> 
mysql> SELECT COUNT(DISTINCT id) FROM score_graph;
+--------------------+
| COUNT(DISTINCT id) |
+--------------------+
|                 18 |
+--------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT COUNT(DISTINCT name) FROM score_graph;                 
+----------------------+
| COUNT(DISTINCT name) |
+----------------------+
|                    3 |
+----------------------+
1 row in set (0.00 sec)

mysql> 
mysql> SELECT COUNT(DISTINCT subject) FROM score_graph;    
+-------------------------+
| COUNT(DISTINCT subject) |
+-------------------------+
|                       6 |
+-------------------------+
1 row in set (0.01 sec)

mysql> 
mysql> SELECT COUNT(DISTINCT score) FROM score_graph;

COUNT(DISTINCT expr,[expr...]) 计算有多少个不重复的expr值，注意是计算非NULL的个数

mysql> SELECT * FROM score_graph;
+----+-----------+---------+-------+
| id | name      | subject | score |
+----+-----------+---------+-------+
|  1 | 尹正杰    | 英语    |   120 |
|  2 | 尹正杰    | 语文    |   118 |
|  3 | 尹正杰    | 数学    |   120 |
|  4 | 尹正杰    | 物理    |   110 |
|  5 | 尹正杰    | 化学    |   112 |
|  6 | 尹正杰    | 生物    |   108 |
|  7 | jason     | 语文    |   120 |
|  8 | jason     | 英语    |   110 |
|  9 | jason     | 数学    |   115 |
| 10 | jason     | 物理    |   105 |
| 11 | jason     | 化学    |   102 |
| 12 | jason     | 生物    |    98 |
| 13 | 耿宇星    | 语文    |   116 |
| 14 | 耿宇星    | 数学    |   112 |
| 15 | 耿宇星    | 英语    |   101 |
| 16 | 耿宇星    | 物理    |   103 |
| 17 | 耿宇星    | 化学    |   112 |
| 18 | 耿宇星    | 生物    |    78 |
+----+-----------+---------+-------+
18 rows in set (0.00 sec)

mysql> 
mysql> SELECT name,MIN(score),MAX(score) FROM score_graph GROUP BY name;
+-----------+------------+------------+
| name      | MIN(score) | MAX(score) |
+-----------+------------+------------+
| 尹正杰    |        108 |        120 |
| jason     |         98 |        120 |
| 耿宇星    |         78 |        116 |
+-----------+------------+------------+
3 rows in set (0.00 sec)

mysql> 
mysql>

MAX([DISTINCT] expr),MIN([DISTINCT] expr) 返回expr中最大或者最小的值

mysql> SELECT * FROM score_graph;
+----+-----------+---------+-------+
| id | name      | subject | score |
+----+-----------+---------+-------+
|  1 | 尹正杰    | 英语    |   120 |
|  2 | 尹正杰    | 语文    |   118 |
|  3 | 尹正杰    | 数学    |   120 |
|  4 | 尹正杰    | 物理    |   110 |
|  5 | 尹正杰    | 化学    |   112 |
|  6 | 尹正杰    | 生物    |   108 |
|  7 | jason     | 语文    |   120 |
|  8 | jason     | 英语    |   110 |
|  9 | jason     | 数学    |   115 |
| 10 | jason     | 物理    |   105 |
| 11 | jason     | 化学    |   102 |
| 12 | jason     | 生物    |    98 |
| 13 | 耿宇星    | 语文    |   116 |
| 14 | 耿宇星    | 数学    |   112 |
| 15 | 耿宇星    | 英语    |   101 |
| 16 | 耿宇星    | 物理    |   103 |
| 17 | 耿宇星    | 化学    |   112 |
| 18 | 耿宇星    | 生物    |    78 |
+----+-----------+---------+-------+
18 rows in set (0.00 sec)

mysql> 
mysql> SELECT name,SUM(score) FROM score_graph GROUP BY name;
+-----------+------------+
| name      | SUM(score) |
+-----------+------------+
| 尹正杰    |        688 |
| jason     |        650 |
| 耿宇星    |        622 |
+-----------+------------+
3 rows in set (0.00 sec)

mysql>

SUM([DISTINCT] expr) 返回expr的求和值

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原文地址：https://www.cnblogs.com/yinzhengjie/p/10335659.html