Mysql两表 join 查询方式

Posted 2023-04-02 大猩猩爱分享

一、SQL基本语法格式

SELECT DISTINCT
	< select_list > 
FROM
	< left_table > < join_type >
JOIN < right_table > ON <join_condition>
WHERE
	< where_condition > 
GROUP BY
	< group_by_list > 
HAVING
	< having_condition > 
ORDER BY
	< order_by_condition > 
LIMIT < limit_number >

二、3种join方式

1. left join（左连接）

A left join B 得到A表的所有字段，如果没有匹配到连接条件则用null填充

select A.*,B.* from A left join B on A.id = B.id;

2. right join（右连接）

A right join B 得到B表所有的字段

select A.*,B.* from A right join B on A.id=B.id;

3. inner join（内连接）

A inner join B得到（A和B的交集）

select A.*,B.* from A inner join B on A.id=B.id;

4. 在理解上面的三种join下，查询（A -  A∩B）

select A.*,B.* from A left join B on A.id=B.id where B.id is null;

 5. 查询 （ B - A∩B ）

select A.*,B.* from A right join B on A.id=B.id where A.id is null;

 6. 查询（A∪B - A∩B）

利用union去重将上面的第四、第五种两条sql中间用union连接即可完成；即先完成一小部分的，然后将两个拼起来的思想。

select A.*,B.* from A left join B on A.id=B.id where B.id is null
union
select A.*,B.* from A right join B on A.id=B.id where A.id is null;

 7. 查询 AUB

mysql中求并集可以使用union关键字进行处理（自动去重）

select A.*,B.* from A left join B on A.id=B.id
UNION
select A.*,B.* from A right join B on A.id=B.id;

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mysql七种常用的JOIN查询

1 A、B两表共有

select * from tbl_emp a inner join tbl_dept b on a.deptId = b.id;

2 A、B两表共有+A的独有

select * from tbl_emp a left join tbl_dept b on a.deptId = b.id;

3 A、B两表共有+B的独有

select * from tbl_emp a right join tbl_dept b on a.deptId = b.id;

4 A的独有

select * from tbl_emp a left join tbl_dept b on a.deptId = b.id where b.id is null;

5 B的独有

select * from tbl_emp a right join tbl_dept b on a.deptId = b.id where a.deptId is null; #B的独有

6 AB全有

#MySQL Full Join的实现 因为MySQL不支持FULL JOIN,下面是替代方法

#left join + union(可去除重复数据)+ right join

SELECT * FROM tbl_emp A LEFT JOIN tbl_dept B ON A.deptId = B.id

UNION

SELECT * FROM tbl_emp A RIGHT JOIN tbl_dept B ON A.deptId = B.id

7 A的独有+B的独有

SELECT * FROM tbl_emp A LEFT JOIN tbl_dept B ON A.deptId = B.id WHERE B.`id` IS NULL

UNION

SELECT * FROM tbl_emp A RIGHT JOIN tbl_dept B ON A.deptId = B.id WHERE A.`deptId` IS NULL;