6,mysql查询语句,select查询

1,select查询的基本结构

select 字段
from 表
where 过滤条件
group by 分组条件
order by 排序条件
having 过滤的第二条件
limit 限定结果条件;

2,最简单的查询语句

mysql> select * from user;
+----+----------+----------+---------------------+
| id | username | password | createtime          |
+----+----------+----------+---------------------+
|  1 | kenan    | kenan    | 2012-10-31 15:32:26 |
|  3 | kenan    | lele     | 2012-10-31 15:32:26 |
+----+----------+----------+---------------------+

3，查询一列或多列

mysql> select username,createtime from user;
+----------+---------------------+
| username | createtime          |
+----------+---------------------+
| kenan    | 2012-10-31 15:32:26 |
| kenan    | 2012-10-31 15:32:26 |
+----------+---------------------+
2 rows in set (0.00 sec)

4,多表查询

mysql> select * from user;
+----+----------+----------+---------------------+
| id | username | password | createtime          |
+----+----------+----------+---------------------+
|  1 | kenan    | kenan    | 2012-10-31 15:32:26 |
|  3 | kenan    | lele     | 2012-10-31 15:32:26 |
+----+----------+----------+---------------------+2 rows in set (0.00 sec)

mysql> select * from teacher;
+----+------------+---------------------+
| id | username   | createtime          |
+----+------------+---------------------+
|  1 | Teacher Li | 2012-11-01 21:00:58 |
|  2 | Teacher Le | 2012-11-01 21:02:07 |
+----+------------+---------------------+
2 rows in set (0.00 sec)

mysql> select * from user,teacher;
+----+----------+----------+---------------------+----+------------+---------------------+
| id | username | password | createtime          | id | username   | createtime          |
+----+----------+----------+---------------------+----+------------+---------------------+
|  1 | kenan    | kenan    | 2012-10-31 15:32:26 |  1 | Teacher Li | 2012-11-01 21:00:58 |
|  3 | kenan    | lele     | 2012-10-31 15:32:26 |  1 | Teacher Li | 2012-11-01 21:00:58 |
|  1 | kenan    | kenan    | 2012-10-31 15:32:26 |  2 | Teacher Le | 2012-11-01 21:02:07 |
|  3 | kenan    | lele     | 2012-10-31 15:32:26 |  2 | Teacher Le | 2012-11-01 21:02:07 |
+----+----------+----------+---------------------+----+------------+---------------------+
4 rows in set (0.00 sec)

多表查询中，如果没有限制条件的话，两个表的记录会分别进行匹配,产生的结果就是两个表的笛卡尔积，叫做全连接
下面的这个带有条件的叫做等同连接，相当于在上面的查询结果中挑选出来满足where后面条件的记录，因为没有所有是空的结果。

mysql> select * from user,teacher where user.username = teacher.username;
Empty set (0.00 sec)

5,带有简单的where字句条件查询

mysql> select * from user where id>2;
+----+----------+----------+---------------------+
| id | username | password | createtime          |
+----+----------+----------+---------------------+
|  3 | kenan    | lele     | 2012-10-31 15:32:26 |
+----+----------+----------+---------------------+
1 row in set (0.00 sec)

6,带有in的查询，in用来表示范围

mysql> select * from user where username in ('kenan','Micheal');
+----+----------+----------+---------------------+
| id | username | password | createtime          |
+----+----------+----------+---------------------+
|  1 | kenan    | kenan    | 2012-10-31 15:32:26 |
|  3 | kenan    | lele     | 2012-10-31 15:32:26 |
+----+----------+----------+---------------------+
2 rows in set (0.00 sec)
这里查询出来username是kenan或者是Micheal的记录

7,between and 用来表示一个字段的范围的查询

mysql> select * from user where id between 2 and 3;
+----+----------+----------+---------------------+
| id | username | password | createtime          |
+----+----------+----------+---------------------+
|  3 | kenan    | lele     | 2012-10-31 15:32:26 |
+----+----------+----------+---------------------+
1 row in set (0.00 sec)

mysql> select * from user where id between 3 and 4;
+----+----------+----------+---------------------+
| id | username | password | createtime          |
+----+----------+----------+---------------------+
|  3 | kenan    | lele     | 2012-10-31 15:32:26 |
+----+----------+----------+---------------------+
1 row in set (0.00 sec)
这里分别查询出来id在2和3之间的记录，3到4之间的记录，从结果可以看出 between and是包含边界的，就是>=2而且<=3

8, is null 查询空值

mysql> select * from user ;
+----+----------+----------+---------------------+
| id | username | password | createtime          |
+----+----------+----------+---------------------+
|  1 | kenan    | kenan    | 2012-10-31 15:32:26 |
|  3 | kenan    | lele     | 2012-10-31 15:32:26 |
|  4 | lele     | lele     | NULL                |
+----+----------+----------+---------------------+
3 rows in set (0.00 sec)

mysql> select * from user where createtime is null;
+----+----------+----------+------------+
| id | username | password | createtime |
+----+----------+----------+------------+
|  4 | lele     | lele     | NULL       |
+----+----------+----------+------------+
1 row in set (0.00 sec)
9,用and连接多条件查询  并关系

mysql> select * from user where username = 'kenan' and password = 'kenan';
+----+----------+----------+---------------------+
| id | username | password | createtime          |
+----+----------+----------+---------------------+
|  1 | kenan    | kenan    | 2012-10-31 15:32:26 |
+----+----------+----------+---------------------+
1 row in set (0.00 sec)

10，用or 的多条件查询 这个是或关系，满足一个条件即可

mysql> select * from user where username = 'kenan' or password = 'kenan';
+----+----------+----------+---------------------+
| id | username | password | createtime          |
+----+----------+----------+---------------------+
|  1 | kenan    | kenan    | 2012-10-31 15:32:26 |
|  3 | kenan    | lele     | 2012-10-31 15:32:26 |
+----+----------+----------+---------------------+
2 rows in set (0.00 sec)

11,聚合函数 count（）用来统计总数

mysql> select count(*) from user;
+----------+
| count(*) |
+----------+
|        3 |
+----------+
1 row in set (0.00 sec)

12，使用sum（）求和，使用avg函数求平均值

mysql> select id from user;
+----+
| id |
+----+
|  1 |
|  3 |
|  4 |
+----+
3 rows in set (0.00 sec)

mysql> select sum(id) from user;
+---------+
| sum(id) |
+---------+
|       8 |
+---------+
1 row in set (0.00 sec)

mysql> select avg(id) from user;
+---------+
| avg(id) |
+---------+
|  2.6667 |
+---------+
1 row in set (0.04 sec)

13，使用max（）求最大值，使用min求最小值

mysql> select id from user;
+----+
| id |
+----+
|  1 |
|  3 |
|  4 |
+----+
3 rows in set (0.00 sec)

mysql> select max(id) from user;
+---------+
| max(id) |
+---------+
|       4 |
+---------+
1 row in set (0.00 sec)

mysql> select min(id) from user;
+---------+
| min(id) |
+---------+
|       1 |
+---------+
1 row in set (0.00 sec)

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