CASE聲明在加入

Question

select col1 
from table1 
case when @col2 is null then left outer join else join end 
table2 on (join condition) 

以上是我的查詢，我想根據1條件選擇左外連接還是右外連接。

是否有落實上述問題的更好的解決方案

Answer 1

select col1 
from table1 
left outer join 
table2 on (join condition) 
where @col2 is null or (@col2 is not null and table2.id is not null) 

這將left outer或inner join之間根據病情選擇。

Answer 2

我不知道這其實是可以以上述方式做...我會寫這樣的事情;所以你根據你的附加條件短路一個JOIN。

select col1 
    from table1 
    left outer join table2 
    on (condition) 
    and @col2 is null 
right outer join table2 
    on (condition) 
    and @col2 is not null 

Answer 3

使用此結構：

select * 
from (
    select 
    Key = 1, 
    -- remainder of left outer join 
    union all 
    select 
    Key=2, 
    -- remainder of right outer join 
) T 
where Key = case when (condition) then 1 else 2 end 

Answer 4

select col1 
from table1 t1 full join table2 t2 on (join condition) 
where case when @col2 is null then t2.col1 else t1.col1 end IS NOT NULL 

你可以試試這個代碼。