sql - SQL 左连接中 "on .. and"和 "on .. where"之间的区别？

Question

这个问题在这里已经有了答案:





What's the difference between where clause and on clause when table left join?

(6 个回答)


去年关闭。




sql语句。

1.select a.* from A a left join B b on a.id =b.id and a.id=2;

2.select a.* from A a left join B b on a.id =b.id where a.id=2;

这两个sql语句有什么区别？

Answer 1

create table A(id int);
create table B(id int);

INSERT INTO A VALUES(1);
INSERT INTO A VALUES(2);
INSERT INTO A VALUES(3);

INSERT INTO B VALUES(1);
INSERT INTO B VALUES(2);
INSERT INTO B VALUES(3);

SELECT * FROM A;
SELECT * FROM B;

id
-----------
1
2
3

id
-----------
1
2
3

过滤 JOIN 以防止在 JOIN 过程中添加行。

select a.*,b.*
from   A a left join B b 
on     a.id =b.id and a.id=2;

id          id
----------- -----------
1           NULL
2           2
3           NULL

WHERE 将在 JOIN 发生后进行过滤。

select a.*,b.* 
from   A a left join B b 
on     a.id =b.id 
where  a.id=2;

id          id
----------- -----------
2           2