PHP之路——MySql查询语句

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  having 过滤的第二条件  
order by 排序条件 
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)

原文链接：http://soukenan.blog.51cto.com/5130995/1047201