MYSQL开发技巧

a表取经四人组 b表悟空的兄弟





1.Join操作的类型–Inner Join

內连接Inner join 基于连接谓词将两张表（如A和B）的列组合在一起，产生新的结果表。

产生的结果集如（图一）红色区域



例如：同时存在与取经四人组中的和悟空的兄弟表中的记录为：

SELECT a.user_name,a.over,b.over FROM a inner JOIN b ON a.user_name=b.user_name

2.Join操作的类型–Left Outer Join

SELECT a.user_name,a.over,b.over FROM a LEFT JOIN  b ON a.user_name=b.user_name;

例：查询取经四人组中哪些不是孙悟空的兄弟

SELECT a.user_name,a.over,b.over FROM a LEFT JOIN b ON a.user_name=b.user_name WHERE b.user_name  is  NULL;

3.Join操作的类型–Right Outer Join

SELECT b.user_name,b.over,a.over FROM a RIGHT JOIN b ON a.user_name=b.user_name ;

例如：悟空的结拜兄弟中哪些没有去取经

SELECT b.user_name,b.over,a.over FROM a RIGHT JOIN b ON a.user_name=b.user_name WHERE a.user_name IS NULL;

4.Join操作的类型–Full Join

SELECT b.'user_name',b.'over',a.'over' FROM a FULL JOIN b ON a.'user_name'=b.'user_name'

在MYSQL中执行会报错，MYSQL**不支持Full Join** 通过UNION ALL实现。

SELECT a.user_name,a.over,b.over FROM a LEFT JOIN b ON a.user_name=b.user_name
UNION ALL
SELECT b.user_name,b.over,a.over FROM a RIGHT JOIN b ON a.user_name=b.user_name

5.Join操作的类型–Corss Join 交叉查询得到的结果为两个表的记录相乘的结果集 笛卡尔基

SELECT a.user_name,a.over,b.user_name,b.over FROM a CROSS JOIN b