表t_a
| id | name |
| 1 | a1 |
| 2 | a2 |
表t_b
| a1_id | name | num |
| 2 | b2 | 1 |
| 3 | b3 | 100 |
left join 后加查询条件
select a.* from t_a a left join t_b b on a.id=b.a1_id and b.num>10
where 1=1
结果：2条数据
where后面加查询条件
select a.* from t_a a left join t_b b on a.id=b.a1_id where b.num>10
结果1条数据
原文：http://www.cnblogs.com/chenjfblog/p/7700188.html