CASE声明在加入

Question

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

以上是我的查询，我想根据1条件选择左外连接还是右外连接。

是否有落实上述问题的更好的解决方案

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) 

这将left outer或inner join之间根据病情选择。

Answer 2

我不知道这其实是可以以上述方式做...我会写这样的事情;所以你根据你的附加条件短路一个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

使用此结构：

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 

你可以试试这个代码。