sql-server - Join 中的 CASE 语句

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 个条件在左外连接或右外连接之间进行选择。

是否有更好的解决方案来实现上述问题

Answer 1

我不确定这实际上是否可以按照所描述的方式进行......我会这样写；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 2

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

你可以试试这段代码。

Answer 3

使用这个结构：

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

这将在left outer或inner join基于条件之间进行选择。