SQL left join 特殊情況

我們通常認爲A left join B後，結果集與A表的記錄條數一致

但，並不是這樣

with  x as 
(
select 1 as id,90 as score from dual union all
select 2 as id,80 as score from dual union all
select 3 as id,70 as score from dual
),
y as (
select 1 as id ,'John' as name from dual union all
select 2 as id,'Adam' as name from dual 
)
select x.id,x.score,y.name from x
left join y on x.id=y.id

 

 這樣的結果是我們預期的，

但如果修改一下，連接條件是id相等，那麼我們嘗試 讓y表的id有兩個相等的

with  x as 
(
select 1 as id,90 as score from dual union all
select 2 as id,80 as score from dual union all
select 3 as id,70 as score from dual
),
y as (
select 1 as id ,'John' as name from dual union all
select 2 as id,'Adam' as name from dual union all
select 2 as id,'Adam2' as name from dual union all
)
select x.id,x.score,y.name from x
left join y on x.id=y.id

 

這時的結果集就是多了一條，所以，在使用left join進行表關聯時

還是很需要注意這個問題的。