MySQL一些常用的高级SQL语句

一、按关键字排序

1、使用ORDER BY语句来实现排序

2、排序可针对一个或多个字段

3、ASC：升序，默认排序方式

4、DESC：降序

5、ORDER BY的语法结构

 语法：select column1，column2，... from 库名 order by column1,column,... asc|desc;

语句使用
mysql -uroot -p123123
mysql> create database score;
mysql> use score;
mysql> create table test(lixing int(6) not null primary key,nianling int(3) not null,xingming char(20) not null,chengji int(3) not null);
mysql> insert into test values(201001,17,'zhangsan',60),(201002,17,'zhaoliu',95),(201003,18,'lisi',70),(201004,18,'wangwu',80),(201005,19,'tianqi',55);
mysql> select * from test;

升序
mysql> select chengji from test order by chengji asc;
+---------+
| chengji |
+---------+
|      55 |
|      60 |
|      70 |
|      80 |
|      95 |
+---------+
5 rows in set (0.00 sec)
mysql> select chengji from test order by chengji;     #默认是ASC
+---------+
| chengji |
+---------+
|      55 |
|      60 |
|      70 |
|      80 |
|      95 |
+---------+
5 rows in set (0.00 sec)

降序
mysql> select chengji from test order by chengji desc;
+---------+
| chengji |
+---------+
|      95 |
|      80 |
|      70 |
|      60 |
|      55 |
+---------+
5 rows in set (0.00 sec)

6、按单字段排序

mysql> select lixing,xingming,chengji from test order by chengji;
+--------+----------+---------+
| xuehao | xingming | chengji |
+--------+----------+---------+
| 201005 | tianqi   |      55 |
| 201001 | zhangsan |      60 |
| 201003 | lisi     |      70 |
| 201004 | wangwu   |      80 |
| 201002 | zhaoliu  |      95 |
+--------+----------+---------+
5 rows in set (0.00 sec)

7、按多字段排序

mysql> select xingming,chengji from test order by nianling desc,chengji desc;
+----------+---------+
| xingming | chengji |
+----------+---------+
| tianqi   |      55 |
| wangwu   |      80 |
| lisi     |      70 |
| zhaoliu  |      95 |
| zhangsan |      60 |
+----------+---------+
5 rows in set (0.00 sec)

二、对结果进行分组

1、使用GROUP BY语句来实现分组

2、通常结合聚合函数一起使用

3、可以按一个或多个字段对结果进行分组

4、GROUP BY分组

mysql> insert into test values(201006,18,'zhangsan',80),(201007,19,'lisi',70);

mysql> select * from test;
+--------+----------+----------+---------+
| xuehao | nianling | xingming | chengji |
+--------+----------+----------+---------+
| 201001 |       17 | zhangsan |      60 |
| 201002 |       17 | zhaoliu  |      95 |
| 201003 |       18 | lisi     |      70 |
| 201004 |       18 | wangwu   |      80 |
| 201005 |       19 | tianqi   |      55 |
| 201006 |       18 | zhangsan |      80 |
| 201007 |       19 | lisi     |      70 |
+--------+----------+----------+---------+
7 rows in set (0.00 sec)

mysql> select count(xingming),nianling from test group by nianling;
+-----------------+----------+
| count(xingming) | nianling |
+-----------------+----------+
|               2 |       17 |
|               3 |       18 |
|               2 |       19 |
+-----------------+----------+
3 rows in set (0.00 sec)

5、GROUP BY结合ORDER BY

mysql> select count(xingming),nianling from test group by nianling order by nianling desc;
+-----------------+----------+
| count(xingming) | nianling |
+-----------------+----------+
|               2 |       19 |
|               3 |       18 |
|               2 |       17 |
+-----------------+----------+
3 rows in set (0.00 sec)

三、限制结果条目

1、只返回select查询结果的第一行或第几行

2、使用limit语句限制条目

3、limit语法结构

语法：
select column1，column2，... from 库名 limit 位置偏移量

mysql> select * from test limit 3;
+--------+----------+----------+---------+
| xuehao | nianling | xingming | chengji |
+--------+----------+----------+---------+
| 201001 |       17 | zhangsan |      60 |
| 201002 |       17 | zhaoliu  |      95 |
| 201003 |       18 | lisi     |      70 |
+--------+----------+----------+---------+
3 rows in set (0.00 sec)

mysql> select * from test limit 3,3;
+--------+----------+----------+---------+
| xuehao | nianling | xingming | chengji |
+--------+----------+----------+---------+
| 201004 |       18 | wangwu   |      80 |
| 201005 |       19 | tianqi   |      55 |
| 201006 |       18 | zhangsan |      80 |
+--------+----------+----------+---------+
3 rows in set (0.00 sec)
#3，3表示从第三行开始数，显示后三行

四、设置别名

1、使用AS语句设置别名，关键字AS可省略

2、设置别名时，保证不能与库中其他表或字段名称冲突

3、别名的语法结构

列的别名：
select 列名 as 列名别名 from 库名；
表的别名：
select 列名 from 库名  as 库名别名；

mysql> select t.xuehao as 学号,t.nianling as 年龄,t.xingming as 姓名,t.chengji as 成绩 from test as t;
+--------+--------+----------+--------+
| 学号   | 年龄   | 姓名     | 成绩   |
+--------+--------+----------+--------+
| 201001 |     17 | zhangsan |     60 |
| 201002 |     17 | zhaoliu  |     95 |
| 201003 |     18 | lisi     |     70 |
| 201004 |     18 | wangwu   |     80 |
| 201005 |     19 | tianqi   |     55 |
| 201006 |     18 | zhangsan |     80 |
| 201007 |     19 | lisi     |     70 |
+--------+--------+----------+--------+
7 rows in set (0.00 sec)

4、as作为连接语句

mysql> create table test1 as select * from test;

mysql> select * from test1;
+--------+----------+----------+---------+
| xuehao | nianling | xingming | chengji |
+--------+----------+----------+---------+
| 201001 |       17 | zhangsan |      60 |
| 201002 |       17 | zhaoliu  |      95 |
| 201003 |       18 | lisi     |      70 |
| 201004 |       18 | wangwu   |      80 |
| 201005 |       19 | tianqi   |      55 |
| 201006 |       18 | zhangsan |      80 |
| 201007 |       19 | lisi     |      70 |
+--------+----------+----------+---------+
7 rows in set (0.00 sec)

五、通配符的使用

1、用于替换字符串中的部分字符

2、通常配合like一起使用，并协同where完成查询

3、常用通配符

（1）、%：表示0个，1个或多个

（2）、_：表示单个字符

mysql> select xuehao,xingming from test where xingming like 'z%';
+--------+----------+
| xuehao | xingming |
+--------+----------+
| 201001 | zhangsan |
| 201002 | zhaoliu  |
| 201006 | zhangsan |
+--------+----------+
3 rows in set (0.00 sec)

mysql> select xuehao,xingming from test where xingming like 'lis_';
+--------+----------+
| xuehao | xingming |
+--------+----------+
| 201003 | lisi     |
| 201007 | lisi     |
+--------+----------+
2 rows in set (0.00 sec)

 六、子查询

6.1、也称作内查询或者嵌套查询

6.2、先于主查询被执行，其结果将作为外层查询的条件

6.3、在增删改查中都可以使用子查询

6.4、支持多层嵌套

6.5、IN语句是用来判断某个值是否在给定的结果集中

6.6、子查询的用法

查询：
mysql> select xuehao as 学号,chengji as 成绩 from test where chengji in (select chengji from test where chengji >=60);
+--------+--------+
| 学号   | 成绩   |
+--------+--------+
| 201001 |     60 |
| 201002 |     95 |
| 201003 |     70 |
| 201007 |     70 |
| 201004 |     80 |
| 201006 |     80 |
+--------+--------+
6 rows in set (0.00 sec)

查询结合降序使用：
mysql> select xuehao as 学号,chengji as 成绩 from test where chengji in (select chengji from test where chengji >=60) order by chengji desc;
+--------+--------+
| 学号   | 成绩   |
+--------+--------+
| 201002 |     95 |
| 201004 |     80 |
| 201006 |     80 |
| 201007 |     70 |
| 201003 |     70 |
| 201001 |     60 |
+--------+--------+
6 rows in set (0.01 sec)

插入：
mysql> create table test_ as select * from score;
mysql> delete from test_;
mysql> select * from test_;
mysql> insert into test_ select * from test where chengji in (select chengji from test where chengji >=80);
mysql> select * from test_;
+--------+----------+----------+---------+
| xuehao | nianling | xingming | chengji |
+--------+----------+----------+---------+
| 201002 |       17 | zhaoliu  |      95 |
| 201004 |       18 | wangwu   |      80 |
| 201006 |       18 | zhangsan |      80 |
+--------+----------+----------+---------+
3 rows in set (0.00 sec)

修改：、
mysql> alter table test_ add column num int(3);
mysql> desc test_ ;
mysql> update test_ set num=101 where chengji in (select chengji from test where chengji >=80);
mysql> select * from test_;
+--------+----------+----------+---------+------+
| xuehao | nianling | xingming | chengji | num  |
+--------+----------+----------+---------+------+
| 201002 |       17 | zhaoliu  |      95 |  101 |
| 201004 |       18 | wangwu   |      80 |  101 |
| 201006 |       18 | zhangsan |      80 |  101 |
+--------+----------+----------+---------+------+
3 rows in set (0.00 sec)

删除：
mysql> delete from test where chengji in(select chengji from (select *from test where chengji >=75)a);
mysql> select * from test;
+--------+----------+----------+---------+
| xuehao | nianling | xingming | chengji |
+--------+----------+----------+---------+
| 201001 |       17 | zhangsan |      60 |
| 201003 |       18 | lisi     |      70 |
| 201005 |       19 | tianqi   |      55 |
| 201007 |       19 | lisi     |      70 |
+--------+----------+----------+---------+
4 rows in set (0.00 sec)

七、NULL值

1、表示缺失的值

2、与数字0或者空白（spaces）是不同的

3、使用IS NULL或IS NOT NULL进行判断

4、NULL值和空值的区别

（1）、空值长度为0，不占空间；NULL值的长度为NULL，占用空间

（2）、IS NULL无法判断空值

（3）、空值使用“=”或者“<>”来处理

（4）、COUNT（）计算时，NULL会忽略，空值会加入计算

插入空值：
mysql> alter table test_ add column class varchar(16);
mysql> select * from test_;
mysql> insert into test_ values(201007,19,'lisi',80,102,'');
mysql> select * from test_;
+--------+----------+----------+---------+------+-------+
| xuehao | nianling | xingming | chengji | num  | class |
+--------+----------+----------+---------+------+-------+
| 201002 |       17 | zhaoliu  |      95 |  101 | NULL  |
| 201004 |       18 | wangwu   |      80 |  101 | NULL  |
| 201006 |       18 | zhangsan |      80 |  101 | NULL  |
| 201007 |       19 | lisi     |      80 |  102 |       |
+--------+----------+----------+---------+------+-------+

null的用法：
mysql> select * from test_ where class is null ;
+--------+----------+----------+---------+------+-------+
| xuehao | nianling | xingming | chengji | num  | class |
+--------+----------+----------+---------+------+-------+
| 201002 |       17 | zhaoliu  |      95 |  101 | NULL  |
| 201004 |       18 | wangwu   |      80 |  101 | NULL  |
| 201006 |       18 | zhangsan |      80 |  101 | NULL  |
+--------+----------+----------+---------+------+-------+
3 rows in set (0.00 sec)
mysql> select * from test_ where class is not null;
+--------+----------+----------+---------+------+-------+
| xuehao | nianling | xingming | chengji | num  | class |
+--------+----------+----------+---------+------+-------+
| 201007 |       19 | lisi     |      80 |  102 |       |
+--------+----------+----------+---------+------+-------+
1 row in set (0.00 sec)

 

 

 八、正则表达式

1、根据指定的匹配模式匹配记录中符合要求的特殊字符

2、使用REGEXP关键字指定匹配模式

3、常用匹配模式

字符	说明
^	匹配开始字符
$	匹配结束字符
.	匹配任意单个字符
*	匹配任意个前面的字符
+	匹配前面字符至少1次
p1|p2	匹配p1或p2
[...]	匹配字符集中括号内的任何字符
[^...]	匹配不在括号内的任何字符
{n}	匹配前面的字符串n次
{n,m}	匹配前面的字符串至少n次，至多m次

 

以z开头的姓名：(^)
mysql> select xuehao,xingming,chengji from test_ where xingming regexp '^z';
+--------+----------+---------+
| xuehao | xingming | chengji |
+--------+----------+---------+
| 201002 | zhaoliu  |      95 |
| 201006 | zhangsan |      80 |
+--------+----------+---------+
2 rows in set (0.00 sec)

以n结尾的姓名：($)
mysql> select xuehao,xingming,chengji from test_ where xingming regexp 'n$';
+--------+----------+---------+
| xuehao | xingming | chengji |
+--------+----------+---------+
| 201006 | zhangsan |      80 |
+--------+----------+---------+
1 row in set (0.00 sec)

匹配单个字符（.）
mysql> select xuehao,xingming,chengji from test_ where xingming regexp 'lis.';
+--------+----------+---------+
| xuehao | xingming | chengji |
+--------+----------+---------+
| 201007 | lisi     |      80 |
+--------+----------+---------+
1 row in set (0.00 sec)

匹配前面字符至少1次（+）
mysql> insert into test_ values(201008,20,'lio',75,103,''),(201009,20,'lioo',85,104,''),(201009,20,'liooo',55,105,'');
mysql> select xuehao,xingming,chengji from test_ where xingming regexp 'lioo+';
+--------+----------+---------+
| xuehao | xingming | chengji |
+--------+----------+---------+
| 201009 | lioo     |      85 |
| 201009 | liooo    |      55 |
+--------+----------+---------+
2 rows in set (0.00 sec)

匹配任意个前面的字符(*)
mysql> select xuehao,xingming,chengji from test_ where xingming regexp 'lio*';
+--------+----------+---------+
| xuehao | xingming | chengji |
+--------+----------+---------+
| 201002 | zhaoliu  |      95 |
| 201007 | lisi     |      80 |
| 201008 | lio      |      75 |
| 201009 | lioo     |      85 |
| 201009 | liooo    |      55 |
+--------+----------+---------+
5 rows in set (0.00 sec)

匹配p1或p2(p1|p2)
mysql> select xuehao,xingming,chengji from test_ where xingming regexp 'l|n';
+--------+----------+---------+
| xuehao | xingming | chengji |
+--------+----------+---------+
| 201002 | zhaoliu  |      95 |
| 201004 | wangwu   |      80 |
| 201006 | zhangsan |      80 |
| 201007 | lisi     |      80 |
| 201008 | lio      |      75 |
| 201009 | lioo     |      85 |
| 201009 | liooo    |      55 |
+--------+----------+---------+
7 rows in set (0.00 sec)

匹配字符集中括号内的任何字符([...])
mysql> select xuehao,xingming,chengji from test_ where xingming regexp '[hz]';
+--------+----------+---------+
| xuehao | xingming | chengji |
+--------+----------+---------+
| 201002 | zhaoliu  |      95 |
| 201006 | zhangsan |      80 |
+--------+----------+---------+
2 rows in set (0.00 sec)

匹配前面的字符串n次{n}
mysql> select xuehao,xingming,chengji from test_ where xingming regexp 'o{2}';
+--------+----------+---------+
| xuehao | xingming | chengji |
+--------+----------+---------+
| 201009 | lioo     |      85 |
| 201009 | liooo    |      55 |
+--------+----------+---------+
2 rows in set (0.00 sec)

匹配前面的字符串至少n次，至多m次({n,m})
mysql> select xuehao,xingming,chengji from test_ where xingming regexp 'o{2,3}';
+--------+----------+---------+
| xuehao | xingming | chengji |
+--------+----------+---------+
| 201009 | lioo     |      85 |
| 201009 | liooo    |      55 |
+--------+----------+---------+
2 rows in set (0.00 sec)

 

 

九、算术运算符

1、MySQL支持的算术运算符

字符	说明
+	加法
-	减法
*	乘法
/	除法
%	取余数

 

mysql> select 5+2 as addition,8-5 as subtraction,6*7 as multiplication,8/2 as division,9%4 as remainder;
+----------+-------------+----------------+----------+-----------+
| addition | subtraction | multiplication | division | remainder |
+----------+-------------+----------------+----------+-----------+
|        7 |           3 |             42 |   4.0000 |         1 |
+----------+-------------+----------------+----------+-----------+
1 row in set (0.00 sec)

 

2、比较运算符

（1）、字符串的比较默认不区分大小写，可使用binary来区分

（2）、常用比较运算符

运算符	说明
=	等于
>	大于
<	小于
>=	大于或等于
<=	小于或等于
!=或<>	不等于
IN	在集合中
LIKE	通配符匹配
IS NULL	判断一个值是否为NULL
IS NOT NULL	判断一个值是否不为NULL
BETWEEN AND	两者之间
GREATEST	两个或多个参数时返回最大值
LEAST	两个或多个参数时返回最小值

 

mysql> select 2=4,2='2','e'='e',(4+4)=(5+3),'n'=NULL;
+-----+-------+---------+-------------+----------+
| 2=4 | 2='2' | 'e'='e' | (4+4)=(5+3) | 'n'=NULL |
+-----+-------+---------+-------------+----------+
|   0 |     1 |       1 |           1 |     NULL |
+-----+-------+---------+-------------+----------+
1 row in set (0.01 sec)

从以上查询可以看出：

①如果两者都是整数，则按整数值进行比较

②如果一个整数一个字符串，则会自动将字符串转换为数字，再进行比较

③如果两者都是字符串，则按照字符串进行比较

④如果两者中至少有一个值是NULL，则比较的结果是NULL

mysql> select 2>4,2<'2', 'e'>='e',(4+4)<=(5+3);
+-----+-------+----------+--------------+
| 2>4 | 2<'2' | 'e'>='e' | (4+4)<=(5+3) |
+-----+-------+----------+--------------+
|   0 |     0 |        1 |            1 |
+-----+-------+----------+--------------+
1 row in set (0.00 sec)

mysql> select 2!=4,null is null,null is not null,2 between 1 and 4;
+------+--------------+------------------+-------------------+
| 2!=4 | null is null | null is not null | 2 between 1 and 4 |
+------+--------------+------------------+-------------------+
|    1 |            1 |                0 |                 1 |
+------+--------------+------------------+-------------------+
1 row in set (0.00 sec)

mysql> select greatest (5,8,12),least (1,5,4);
+-------------------+---------------+
| greatest (5,8,12) | least (1,5,4) |
+-------------------+---------------+
|                12 |             1 |
+-------------------+---------------+
1 row in set (0.00 sec)

十、逻辑运算符

1、又称为布尔运算符

2、用来判断表达式的真假

3、常用的逻辑运算符

运算符	说明
NOT或！	逻辑非
AND或&&	逻辑与
OR或||	逻辑或
XOR	逻辑异或

逻辑非
mysql> select not 2,!3,not 0,!(4-4);
+-------+----+-------+--------+
| not 2 | !3 | not 0 | !(4-4) |
+-------+----+-------+--------+
|     0 |  0 |     1 |      1 |
+-------+----+-------+--------+
1 row in set (0.00 sec)

逻辑与
mysql> select 2 and 3,4 && 0,0 && NULL,1 and NULL;
+---------+--------+-----------+------------+
| 2 and 3 | 4 && 0 | 0 && NULL | 1 and NULL |
+---------+--------+-----------+------------+
|       1 |      0 |         0 |       NULL |
+---------+--------+-----------+------------+
1 row in set (0.00 sec)

十一、位运算符

11.1、对二进制数进行计算的运算符

11.2、常用的位运算符

运算符	说明
&	按位与
|	按位或
~	按位取反
^	按位异或
<<	按位左移
>>	按位右移

mysql> select 4&5,4|5,4&~3,3^4,2<<2,2>>1;
+-----+-----+------+-----+------+------+
| 4&5 | 4|5 | 4&~3 | 3^4 | 2<<2 | 2>>1 |
+-----+-----+------+-----+------+------+
|   4 |   5 |    4 |   7 |    8 |    1 |
+-----+-----+------+-----+------+------+
1 row in set (0.00 sec)

十二、连接查询

MySQL的连接查询，通常都是将来自两个或多个表的行结合起来，基于这些表之间的共同字段，进行数据的拼接，首先，要确定一个主表作为结果集，然后将其他表的行有选择性的连接到选定的主表结果集上，使用较多的连接查询包括：内连接、左连接和右连接

1、内连接及示意图

2、左连接及示意图

mysql> select t.xuehao,t.xingming,t.chengji from test_ t left join test t1 on t.xingming=t1.xingming;
+--------+----------+---------+
| xuehao | xingming | chengji |
+--------+----------+---------+
| 201006 | zhangsan |      80 |
| 201007 | lisi     |      80 |
| 201007 | lisi     |      80 |
| 201002 | zhaoliu  |      95 |
| 201004 | wangwu   |      80 |
| 201008 | lio      |      75 |
| 201009 | lioo     |      85 |
| 201009 | liooo    |      55 |
+--------+----------+---------+
8 rows in set (0.00 sec)

2、右连接及示意图

mysql> select t.xuehao,t.xingming,t.chengji from test_ t right join test t1 on t.xingming=t1.xingming;
+--------+----------+---------+
| xuehao | xingming | chengji |
+--------+----------+---------+
| 201006 | zhangsan |      80 |
| 201007 | lisi     |      80 |
| 201007 | lisi     |      80 |
|   NULL | NULL     |    NULL |
+--------+----------+---------+
4 rows in set (0.00 sec)

十三、函数

1、数学函数

函数	含义
abs(x)	返回x的绝对值
rand()	返回0到1的随机数
mod(x,y)	返回x除以y以后的余数
power(x,y)	返回x的y次方
round(x)	返回离x最近的整数
round(x,y)	保留x的y位小数四舍五入后的值
sqrt(x)	返回x的平方根
truncate(x,y)	返回数字x截断为y位小树的值
ceil(x)	返回大于或等于x的最小整数
floor(x)	返回小于或等于x的最大整数
greatest(x1,x2...)	返回集合中最大的值
least(x1,x2...)	返回集合中最小的值

常用的数学函数举例说明 1 mysql> select abs(-12),rand(),rand(),mod(4,5),power(2,6);
 2 +----------+--------------------+--------------------+----------+------------+
 3 | abs(-12) | rand()             | rand()             | mod(4,5) | power(2,6) |
 4 +----------+--------------------+--------------------+----------+------------+
 5 |       12 | 0.7755411516647238 | 0.7329282013074757 |        4 |         64 |
 6 +----------+--------------------+--------------------+----------+------------+
 7 1 row in set (0.00 sec)
 8
 9 mysql> select round(2.4),round(2.5),round(2.4235,2),sqrt(2),truncate(2.4652,2);
10 +------------+------------+-----------------+--------------------+--------------------+
11 | round(2.4) | round(2.5) | round(2.4235,2) | sqrt(2)            | truncate(2.4652,2) |
12 +------------+------------+-----------------+--------------------+--------------------+
13 |          2 |          3 |            2.42 | 1.4142135623730951 |               2.46 |
14 +------------+------------+-----------------+--------------------+--------------------+
15 1 row in set (0.00 sec)
16
17 mysql> select ceil(2.2),floor(2.8),greatest(1,2,3,4,5),least(1,2,3,4,5);
18 +-----------+------------+---------------------+------------------+
19 | ceil(2.2) | floor(2.8) | greatest(1,2,3,4,5) | least(1,2,3,4,5) |
20 +-----------+------------+---------------------+------------------+
21 |         3 |          2 |                   5 |                1 |
22 +-----------+------------+---------------------+------------------+
23 1 row in set (0.00 sec)

2、聚合函数：对表中数据记录进行集中概括而设计的一类函数

函数	含义
avg()	返回指定列的平均值
count()	返回指定列中非NULL值的个数
min()	返回指定看列的最小值
max()	返回指定列的最大值
sum()	返回指定列的所有值之和

聚合函数举例
mysql> select avg(chengji) from test;
+--------------+
| avg(chengji) |
+--------------+
|      63.7500 |
+--------------+
1 row in set (0.00 sec)

mysql> select sum(chengji) from test;
+--------------+
| sum(chengji) |
+--------------+
|          255 |
+--------------+
1 row in set (0.00 sec)

mysql> select min(chengji) from test;
+--------------+
| min(chengji) |
+--------------+
|           55 |
+--------------+
1 row in set (0.00 sec)

mysql> select max(chengji) from test;
+--------------+
| max(chengji) |
+--------------+
|           70 |
+--------------+
1 row in set (0.00 sec)

mysql> alter table test add sex char(2);
mysql> select * from test;
+--------+----------+----------+---------+------+
| xuehao | nianling | xingming | chengji | sex  |
+--------+----------+----------+---------+------+
| 201001 |       17 | zhangsan |      60 | 男   |
| 201003 |       18 | lisi     |      70 | NULL |
| 201005 |       19 | tianqi   |      55 | NULL |
| 201007 |       19 | lisi     |      70 | NULL |
+--------+----------+----------+---------+------+
4 rows in set (0.00 sec)
mysql> update test set sex='男' where xingming='lisi';
mysql> update test set sex='男' where xingming='zhangsan';
mysql> select * from test;
+--------+----------+----------+---------+------+
| xuehao | nianling | xingming | chengji | sex  |
+--------+----------+----------+---------+------+
| 201001 |       17 | zhangsan |      60 | 男   |
| 201003 |       18 | lisi     |      70 | 男   |
| 201005 |       19 | tianqi   |      55 | NULL |
| 201007 |       19 | lisi     |      70 | 男   |
+--------+----------+----------+---------+------+
4 rows in set (0.00 sec)
mysql> select count(sex) from test;
+------------+
| count(sex) |
+------------+
|          3 |
+------------+
1 row in set (0.00 sec)

3、字符串函数

函数	含义
length(x)	返回字符串x的长度
trim()	返回去除指定格式的值
concat(x,y)	将提供的参数x和y拼接成一个字符串
upper(x)	将字符串x的所有字母变成大写字母
lower(x)	将字符串x的所有字母变成小写字母
left(x,y)	返回字符串x的前y个字符
right(x,y)	返回字符串x的后y个字符
repeat(x.y)	将字符串x重复y次
space(x)	返回x个空格
replace(x,y,z)	将字符串z替代字符串x中的字符串y
strcmp(x,y)	比较x和y，返回的值可以位-1，0，1
substring(x,y,z)	获取从从字符串x中的第y个位置开始长度为z的字符串
reverse(x)	将字符串x反转

举例说明
mysql> select length('abc'),trim(  'ab' ),concat('ab','cd'),upper('abc'),lower('ABC');
+---------------+---------------+-------------------+--------------+--------------+
| length('abc') | trim(  'ab' ) | concat('ab','cd') | upper('abc') | lower('ABC') |
+---------------+---------------+-------------------+--------------+--------------+
|             3 | ab            | abcd              | ABC          | abc          |
+---------------+---------------+-------------------+--------------+--------------+
1 row in set (0.00 sec)

mysql> select left('abcd',2),right('abcd',2),repeat('abc',3);
+----------------+-----------------+-----------------+
| left('abcd',2) | right('abcd',2) | repeat('abc',3) |
+----------------+-----------------+-----------------+
| ab             | cd              | abcabcabc       |
+----------------+-----------------+-----------------+
1 row in set (0.00 sec)

mysql> select space(2),replace('abcde','e','g'),strcmp('a','b'),strcmp('a','a'),strcmp('b','a');
+----------+--------------------------+-----------------+-----------------+-----------------+
| space(2) | replace('abcde','e','g') | strcmp('a','b') | strcmp('a','a') | strcmp('b','a') |
+----------+--------------------------+-----------------+-----------------+-----------------+
|          | abcdg                    |              -1 |               0 |               1 |
+----------+--------------------------+-----------------+-----------------+-----------------+
1 row in set (0.00 sec)

mysql> select substring('abcdefg',3,3),reverse('abcdefg');
+--------------------------+--------------------+
| substring('abcdefg',3,3) | reverse('abcdefg') |
+--------------------------+--------------------+
| cde                      | gfedcba            |
+--------------------------+--------------------+
1 row in set (0.00 sec)

4、日期时间函数

函数	含义
curdate()	返回当前时间的年月日
curtime()	返回当前时间的时分秒
now()	返回当前时间的日期和时间
month()	返回日期x中的月份值
week(x)	返回日期x是年度第几个星期
hour(x)	返回x中的小时值
minute(x)	返回x中的分钟值
second(x)	返回x中的秒钟值
dayofweek(x)	返回x是星期几，1星期日，2星期1
dayofmonth(x)	计算日期x是本月的第几天
dayofyear(x)	计算日期x是本年的第几天

mysql> select curdate(),curtime(),now(),month('2020-08-15'),week('2020-08-15');
+------------+-----------+---------------------+---------------------+--------------------+
| curdate()  | curtime() | now()               | month('2020-08-15') | week('2020-08-15') |
+------------+-----------+---------------------+---------------------+--------------------+
| 2020-11-02 | 19:03:03  | 2020-11-02 19:03:03 |                   8 |                 32 |
+------------+-----------+---------------------+---------------------+--------------------+
1 row in set (0.00 sec)

mysql> select hour('20:15:45'),minute('20:15:45'),second('20:15:45');
+------------------+--------------------+--------------------+
| hour('20:15:45') | minute('20:15:45') | second('20:15:45') |
+------------------+--------------------+--------------------+
|               20 |                 15 |                 45 |
+------------------+--------------------+--------------------+
1 row in set (0.00 sec)

mysql> select dayofweek('2020-08-15'),dayofmonth('2020-08-15'),dayofyear('2020-08-15');
+-------------------------+--------------------------+-------------------------+
| dayofweek('2020-08-15') | dayofmonth('2020-08-15') | dayofyear('2020-08-15') |
+-------------------------+--------------------------+-------------------------+
|                       7 |                       15 |                     228 |
+-------------------------+--------------------------+-------------------------+
1 row in set (0.00 sec)

十四、存储过程

1、简介

（1）、是一组为了完成特定功能的SQL语句集合

（2）、比传统的SQL速度更快、执行效率更高

（3）、存储过程的优点

①执行一次后，会将生成的二进制代码驻留缓冲区，提高执行效率

②SQL语句加上控制语句的集合，灵活性高

③在服务器端存储，客户端调用时，降低网络负载

④可多次重复被调用，可随时修改，不影响客户端调用

⑤可完成所有的数据库操作，也可控制数据库的信息访问权限

2、创建存储过程

（1）、使用CREATE PROCEDURE语句创建存储过程

（2）、创建存储过程的语法结构

CREATE PROCEDURE <过程名> （过程参数[...]）<过程体> [过程参数[...]] 格式 [IN|OUT|INOUT] <参数名> <类型>

3、参数分为

（1）、输入参数：IN

（2）、输出参数：OUT

（3）、输入/输出参数：INOUT

4、存储过程的主体部分，被称为过程体

5、以BEGIN开始，以END结束，若只有一条SQL语句

6、以DELIMITER开始和结束

7、存储过程

mysql> delimiter $$
mysql> create procedure a()
    -> begin
    -> select * from test limit 3;
    -> end $$
Query OK, 0 rows affected (0.00 sec)

mysql> delimiter ;
mysql> call a();
+--------+----------+----------+---------+------+
| xuehao | nianling | xingming | chengji | sex  |
+--------+----------+----------+---------+------+
| 201001 |       17 | zhangsan |      60 | 男   |
| 201003 |       18 | lisi     |      70 | 男   |
| 201005 |       19 | tianqi   |      55 | NULL |
+--------+----------+----------+---------+------+
3 rows in set (0.01 sec)

Query OK, 0 rows affected (0.01 sec)

mysql> use score;
mysql> set @num1=1,@num2=2,@num3=3;
mysql> delimiter $$
mysql> create procedure p(in num1 int,out num2 int,inout num3 int)
    -> begin
    -> select num1,num2,num3;
    -> set num1=10,num2=20,num3=30;
    -> select num1,num2,num3;
    -> end $$
Query OK, 0 rows affected (0.00 sec)

mysql> delimiter ;
mysql> call p(@num1,@num2,@num3);
+------+------+------+
| num1 | num2 | num3 |
+------+------+------+
|    1 | NULL |    3 |
+------+------+------+
1 row in set (0.00 sec)

+------+------+------+
| num1 | num2 | num3 |
+------+------+------+
|   10 |   20 |   30 |
+------+------+------+
1 row in set (0.00 sec)
总结1：in和inout参数会将全局变量的值传入存储过程中，而out参数不会将全局变量的值传入存储过程中，在全局过程使用中，参数值in、out、inout都会发生改变

mysql> select @num1,@num2,@num3;
+-------+-------+-------+
| @num1 | @num2 | @num3 |
+-------+-------+-------+
|     1 |    20 |    30 |
+-------+-------+-------+
1 row in set (0.00 sec)
总结2：调用完存储过程后，发现in参数不会对全局变量的值引起变化，而out和inout参数调用完存储过程后，会对全局变量的值产生变化，会将存储过程引用后的值赋值给全局变量，in参数赋值类型可以是变量还有定值，而out和inout参数赋值类型必须是变量

8、修改存储过程

（1）、存储过程的修改分为特征修改和内容修改

（2）、特征修改的方法

  ALTER PROCEDURE <过程名> [<特征>...]

（3）、内容修改可先删除原有存储过程，之后再创建方法

9、删除存储过程

（1）、删除存储过程的语法

  DROP { PROCEDURE | FUNCTION } [ IF EXISTS ] <过程名>

（2）、删除的过程

mysql> drop procedure a;
mysql> call a();
ERROR 1305 (42000): PROCEDURE score.a does not exist