left join条件写在on中和where中的区别

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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

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

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