mysql 检索信息

select语句用来从数据表中检索信息；其一般格式为：

SELECT what_to_select FROW which_table WHERE conditions_to_satisfy;

what_to_select ：指出你想要看见的内容，可以是列的一个表，或*表示“所有的列”。

which_table ：指出你要检索数据的表。

conditions_to_satisfy :WHERE子句是可选项，如果选择该项，则conditions_to_satify指定行必须满足的检索条件。

1，查看整张表的数据

mysql>select * from student;
+----+-------+-------+------+------+-------------+--------+--------+---------------------+
| id | fName | lName | age  | sex  | phone       | city   | state  | createDate          |
+----+-------+-------+------+------+-------------+--------+--------+---------------------+
|  1 | 四    |  李    |   22 | 女   | 13223231123 |  成都市 |  四川省 | 2017-05-29 00:00:00 |
|  2 | 五    |  王    |   33 | 男   | 13900002222 |  成都市 |  四川省 | 2017-05-29 00:00:00 |
|  3 | Joy   | Mark  |   18 | 女   | 17620201010 |  成都市 |  四川省 | 2017-05-29 00:00:00 |
|  4 | Wong  | Kiwin |   18 | 男   | 13528282323 |  成都市 |  四川省 | 2017-05-29 00:00:00 |
+----+-------+-------+------+------+-------------+--------+--------+---------------------+
4 rows in set (0.00 sec)

2，查看指定行的数据(查看的是lName列中‘李’所在行的整行数据)

mysql> select * from student where lName='李';
+----+-------+-------+------+------+-------------+--------+--------+---------------------+
| id | fName | lName | age  | sex  | phone       | city   | state  | createDate          |
+----+-------+-------+------+------+-------------+--------+--------+---------------------+
|  1 | 四    |  李    |   22 | 女   | 13223231123 | 成都市  | 四川省  | 2017-05-29 00:00:00 |
+----+-------+-------+------+------+-------------+--------+--------+---------------------+
1 row in set (0.00 sec)

 3，查看指定列的数据(查看的是lName列的整列数据)

mysql> select lName from student;
+-------+
| lName |
+-------+
| 李    |
| 王    |
| Mark  |
| Kiwin |
+-------+
4 rows in set (0.00 sec)

 4，查看具体位置的数据(查看的是lName列中的'王')

mysql> select lName from student where lName='王';
+-------+
| lName |
+-------+
|  王   |
+-------+
1 row in set (0.00 sec)

 5，查看满足一定条件限制的数据(查看的是age>20对应的fName和lName的数据)

mysql> select fName,lName from student where age>20;
+-------+-------+
| fName | lName |
+-------+-------+
|  四    | 李    |
|  五    | 王    |
+-------+-------+
2 rows in set (0.00 sec)

 6,查看满足组合条件的数据（age>20 的 女生）

mysql> select fName,lName from student where (age>20 and sex='女');
+-------+-------+
| fName | lName |
+-------+-------+
|  四   |  李    |
+-------+-------+
1 row in set (0.00 sec)

 7,去除重复数据的关键字：distinct(当distinct关键字同时修饰几个字段时，以重复数据最少的字段为准)

mysql> select distinct createDate from student;
+---------------------+
| createDate          |
+---------------------+
| 2017-05-29 00:00:00 |
+---------------------+
1 row in set (0.00 sec)
mysql> select distinct sex from student;
+------+
| sex  |
+------+
| 女   |
| 男   |
+------+
2 rows in set (0.00 sec)
mysql> select distinct sex,lName from student;
+------+-------+
| sex  | lName |
+------+-------+
| 女   | 李    |
| 男   | 王    |
| 女   | Mark  |
| 男   | Kiwin |
+------+-------+
4 rows in set (0.00 sec)

mysql> select distinct sex,createDate from student;
+------+---------------------+
| sex  | createDate          |
+------+---------------------+
| 女   | 2017-05-29 00:00:00 |
| 男   | 2017-05-29 00:00:00 |
+------+---------------------+
2 rows in set (0.00 sec)

mysql> select distinct createDate,sex from student;
+---------------------+------+
| createDate          | sex  |
+---------------------+------+
| 2017-05-29 00:00:00 | 女   |
| 2017-05-29 00:00:00 | 男   |
+---------------------+------+
2 rows in set (0.00 sec)

 8，以某个字段排序：order by(binary区分大小写)字段 desc(降序)/asc(升序)

mysql> select fName,age,createDate from student order by age;//默认为升序
+-------+------+---------------------+
| fName | age  | createDate          |
+-------+------+---------------------+
| Joy   |   18 | 2017-05-29 00:00:00 |
| Wong  |   18 | 2017-05-29 00:00:00 |
| 四    |   22 | 2017-05-29 00:00:00 |
| 五    |   33 | 2017-05-29 00:00:00 |
+-------+------+---------------------+
4 rows in set (0.00 sec)

mysql> select fName,age,createDate from student order by age desc;
+-------+------+---------------------+
| fName | age  | createDate          |
+-------+------+---------------------+
| 五    |   33 | 2017-05-29 00:00:00 |
| 四    |   22 | 2017-05-29 00:00:00 |
| Joy   |   18 | 2017-05-29 00:00:00 |
| Wong  |   18 | 2017-05-29 00:00:00 |
+-------+------+---------------------+
4 rows in set (0.00 sec)
//其中会先将age按照默认的升序排列，再在age相同的范围内对createDate按降序排序
mysql> select age,phone,createDate from student order by age,createDate desc;
+------+-------------+---------------------+
| age  | phone       | createDate          |
+------+-------------+---------------------+
|   18 | 17620201010 | 2015-09-19 00:00:00 |
|   18 | 13528282323 | 2007-12-30 00:00:00 |
|   22 | 13223231123 | 2017-05-29 00:00:00 |
|   33 | 13900002222 | 2016-08-20 00:00:00 |
+------+-------------+---------------------+
4 rows in set (0.00 sec)
//先将age按默认升序排列，再在age相同的情况下对createDate按降序排序，最后在createDate相同的情况下对phone按升序排序
mysql> select age,phone,createDate from student order by age,createDate desc,phone asc;
+------+-------------+---------------------+
| age  | phone       | createDate          |
+------+-------------+---------------------+
|   18 | 13223231123 | 2017-05-29 00:00:00 |
|   18 | 17620201010 | 2017-05-29 00:00:00 |
|   18 | 13528282323 | 2007-12-30 00:00:00 |
|   33 | 13900002222 | 2016-08-20 00:00:00 |
+------+-------------+---------------------+
4 rows in set (0.00 sec)

 9，日期计算

mysql> SELECT name, birth, CURDATE(),
 -> (YEAR(CURDATE())-YEAR(birth))
 -> - (RIGHT(CURDATE(),5)<RIGHT(birth,5))
 -> AS age
 -> FROM pet;
+----------+------------+------------+------+
|   name   |    birth   |  CURDATE() |  age |
+----------+------------+------------+------+
|  Fluffy  | 1993-02-04 | 2003-08-19 |  10  |
|  Claws   | 1994-03-17 | 2003-08-19 |   9  |
|  Buffy   | 1989-05-13 | 2003-08-19 |  14  |
|  Fang    | 1990-08-27 | 2003-08-19 |  12  |
|  Bowser  | 1989-08-31 | 2003-08-19 |  13  |
|  Chirpy  | 1998-09-11 | 2003-08-19 |   4  |
|  Whistler| 1997-12-09 | 2003-08-19 |   5  |
|  Slim    | 1996-04-29 | 2003-08-19 |   7  |
|  Puffball| 1999-03-30 | 2003-08-19 |   4  |
+----------+------------+------------+------+
此处，YEAR()提取日期的年部分，RIGHT()提取日期的MM-DD (日历年)部分的最右面5个字符。比较MM-DD值
的表达式部分的值一般为1或0，如果CURDATE()的年比birth的年早，则年份应减去1。整个表达式有些难懂，
使用alias (age)来使输出的列标记更有意义。