TSQL根据CASE值从不同表中选择

Question

我在SQL Server 2012上运行.

想象一下,我有3个表(表1,表2,表3),它们共有2个字段(ID,WhereParam).

实现这一目标的正确方法是什么？

SELECT ID FROM 
            (CASE @Var  
                WHEN "AA" THEN Table1  
                WHEN "BB" THEN Table2  
                ELSE Table3)
      WHERE "TableSelected".WhereParam = @WhereParam

Answer 1

使用IF(正确的方式)

IF @Var = 'AA'
BEGIN
    SELECT ID FROM Table1
    -- WHERE ...
END
ELSE IF @Var = 'BB'
BEGIN
    SELECT ID FROM Table2 
    -- WHERE ...
END
ELSE
BEGIN
    SELECT ID FROM Table3  
    -- WHERE ...
END

其他方式(你应该避免但我会发布以供参考)

使用动态SQL

DECLARE @sql NVARCHAR(MAX)
SELECT @sql = 'SELECT ID FROM ' + CASE @Var WHEN 'AA' THEN 'Table1' 
                                            WHEN 'BB' THEN 'Table2' ELSE 'Table3' END + ' -- WHERE ...'
EXEC(@sql)

运用 UNION

SELECT Id
FROM Table1
WHERE @Var='AA'
-- AND ...

UNION ALL

SELECT Id
FROM Table2
WHERE @Var='BB'
-- AND ...

UNION ALL

SELECT Id
FROM Table3
WHERE @Var NOT IN('AA', 'BB')
-- AND ...

Answer 2

一般来说,您无法在静态SQL中动态指定表.但是,您可以通过几种方式获得相同的结果.以下应该有效:

Select ID from (
   Select ID, WhereParam from Table1 where @Var = 'AA'
   union all
   Select ID, WhereParam from Table2 where @Var = 'BB'
   union all
   Select ID, WhereParam from Table3 where @Var not in ('AA','BB'))
where WhereParam = @WhereParam