left join条件放在on后面和where后面的区别

on后面的条件：

• 对于左表来说，不管on的条件是否为真，左表都会全部返回
• 对于右表来说，当on的条件为假时，左表数据用null返回

where后面的条件：

• 对连接后的结果表进行过滤 

如果将右表的过滤条件放在了where中，就可能会出现左连接后部分数据丢失的情况。

举个例子

有两个表，leader和department

 

下面看这两个SQL执行结果的区别

select * from department d LEFT JOIN leader l on d.departmentID=l.departmentID and l.departmentID is not null;

 

select * from department d LEFT JOIN leader l on d.departmentID=l.departmentID where l.departmentID is not null; 

 

再看下面这两个SQL的区别

select * from leader l LEFT JOIN department d on l.departmentID=d.departmentID and l.departmentID is not null;
 

select * from leader l LEFT JOIN department d on l.departmentID=d.departmentID where l.departmentID is not null;