left join 过滤条件写在on后面和写在where 后面的区别

create table t1(id int, feild int);
insert into t1 values(1 , 1);
insert into t1 values(1 , 2);
insert into t1 values(1 , 3);
insert into t1 values(1 , 4);
insert into t1 values(2 , 1);
insert into t1 values(2 , 2);
create table t2(id int, feild int);
insert into t2 values(1 , 1);
insert into t2 values(1 , 2);
insert into t2 values(1 , 5);
insert into t2 values(1 , 6);
insert into t2 values(2 , 1);
insert into t2 values(2 , 3);
select t1.*,t2.* from t1 left join t2 on t1.id=t2.id  

--取t1表的第一行,扫瞄t2表,按条件做对比,如果满足条件,就加入返回结果表. 然后取t1表的第二行,扫瞄t2表,按条件做对比,如果满足条件,就加入返回结果表. 重复以上过程,直到t1表扫描结束.

select t1.*,t2.* from t1 left join t2 on t1.id=t2.id and t1.feild=1

--给左表加条件的时候,左表满足条件的,按上面的过程返回值,左表不满足条件的,直接输出,右表的列补null 1 1 1 1 1 1 1 2 1 1 1 5 1 1 1 6 2 1 2 1 2 1 2 3 1 2 NULL NULL 1 3 NULL NULL 1 4 NULL NULL 2 2 NULL NULL

select t1.*,t2.* from t1 left join t2 on t1.id=t2.id where t1.feild=1 先执行where后连接查询，执行where后t1表为

1 , 1

2 , 1

用它来left join t2.

--下面三条语句查询结果是一样的,当为右表加条件的时候,可以把left join 改为inner jin, 因为inner join比left join 要快!

select t1.*,t2.* from t1 left join t2 on t1.id=t2.id  and t2.feild=1
select t1.*,t2.* from t1 left join t2 on t1.id=t2.id  where t2.feild=1 
select t1.*,t2.* from t1 inner join t2 on t1.id=t2.id  and t2.feild=1