sql中left join on where区别剖析

select from tb1 left join tb2 on tb1.xx=tb2.xx and tb2.xxxx=5
先筛选tb2.xxxx=5 再把tb1与筛选后的临时表进行左连接。

select from tb1 left join tb2 on tb1.xx=tb2.xx where tb2.xxxx=5
先把tb1与tb2进行左连接，连接后产生的临时表筛选出tb2.xxxx=5的记录。

SELECT * FROM tb1;
id name age
------ ------- --------
1 liuchao 34
2 lilijun 28
3 chengyu 44

SELECT * FROM tb2; id name task
------ ------- --------
1 liuchao work
2 liuchao study
3 lilijun study

SELECT * FROM tb1 LEFT JOIN tb2 ON tb1.name=tb2.name AND tb2.`name`='liuchao'; id name age id name task
------ ------- ------ ------ ------- --------
1 liuchao 34 1 liuchao work
1 liuchao 34 2 liuchao study
2 lilijun 28 (NULL) (NULL) (NULL)
3 chengyu 44 (NULL) (NULL) (NULL)

SELECT * FROM tb1 LEFT JOIN tb2 ON tb1.name=tb2.name WHERE tb2.`task` IS NOT NULL; id name age id name task
------ ------- ------ ------ ------- --------
1 liuchao 34 1 liuchao work
1 liuchao 34 2 liuchao study
2 lilijun 28 3 lilijun study