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,掃瞄t2表,按條件做對比,如果滿足條件,就加入返回結果表.
   然後取t1表的第二行,掃瞄t2表,按條件做對比,如果滿足條件,就加入返回結果表.
   重複以上過程,直到t1表掃描結束.

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

--給左表加條件的時候,左表滿足條件的,按上面的過程返回值,左表不滿足條件的,直接輸出,右表的列補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     先執行where後連線查詢

                                                                                    執行where後表為   1 , 1
                                                                                                                      2 , 1
                                                                                   用它來left join t2.
 

 --下面三條語句查詢結果是一樣的,當為右表加條件的時候,可以把left join 改為inner jin, 因為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

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