Question

我的查询如下：

select * 
from BASE_TABLE
inner join (
    select * from TABLE_A
) t on BASE_TABLE.ID = t.ID

并希望使嵌套查询成为条件，例如：

select * 
from BASE_TABLE
inner join (
    if @var = 1 
        select * from TABLE_A
    else 
        select * from TABLE_B
) t on BASE_TABLE.ID = t.ID

有可能吗？查询在存储过程中使用。解决方案应该适用于SQL Server和Oracle，或者至少适用于SQL Server。

Answer 1

这个怎么样：

SELECT
  * 
FROM
  BASE_TABLE
  INNER JOIN (
    SELECT col1, col2, colN FROM TABLE_A WHERE @var IN (1) -- "if"
    UNION
    SELECT col1, col2, colN FROM TABLE_B WHERE @var IN (2) -- "else if"
    UNION
    SELECT col1, col2, colN FROM TABLE_C WHERE @var NOT IN (1, 2) -- "else"
  ) t ON t.ID = BASE_TABLE.ID

Answer 2

我会这样做（但是你需要指定字段;你不能使用select *语法）：

select base.ID, base.Field1, base.Etc, Field1 = case when @var=1 then a.Field1 else b.Field1 end,
       Field2 = case when @var=1 then a.Field2 else b.Field2 end
  from BASE_TABLE base
  left join TABLE_A a on base.ID = a.ID
  left join TABLE_B b on base.ID = b.ID

Answer 3

请尝试以下查询

declare @var int=1  
declare @qry nvarchar(max)  
set @qry='select * from BASE_TABLE ('  
if @var=1  
set @qry=@qry+ 'select * from TABLE_A'  
else  
set @qry=@qry+ 'select * from TABLE_B'  

set @qry=@qry+')t on BASE_TABLE.ID = t.ID'

exec (@qry)

SQL条件嵌套查询

3 个答案: