sql - 按不同表中的案例进行分组

Question

我正在尝试按案例陈述的结果进行分组。

当我按同一个表中的值进行分组时，它可以正常工作:

Select 
   Case 
      When @Col = 1 Then YEAR(tbl1.myDate1)
      When @Col = 2 Then YEAR(tbl1.myDate2)
   End as [Year],
   SUM(tbl1.myValue) as [Total]
From tabel1 as tbl1
Group By Case 
      When @Col = 1 Then YEAR(tbl1.myDate1)
      When @Col = 2 Then YEAR(tbl1.myDate2)
   End

但是当我在 case 语句中插入来自不同表的值时，它会崩溃并出现如下错误:

tabel1.myDate1 is not contained in an aggregate function or group by clause.

tabel1.myDate2 is not contained in an aggregate function or group by clause.

Select 
   Case 
      When @Col = 1 Then YEAR(tbl1.myDate1)
      When @Col = 2 Then YEAR(tbl1.myDate2)
      When @Col = 3 Then YEAR(tbl2.myDate)
   End as [Year],
   SUM(tbl1.myValue) as [Total]
From tabel1 as tbl1
Left Join tabel2 as tbl2
    On tbl1.tbl2ID = tbl2.ID
Group By Case 
      When @Col = 1 Then YEAR(tbl1.myDate1)
      When @Col = 2 Then YEAR(tbl1.myDate2)
      When @Col = 3 Then YEAR(tbl2.myDate)
   End

我很好奇为什么会出现此错误，因为这两个字段显然都包含在 group by 子句中。

此外，第一个代码示例表明按大小写分组是可以的。 为什么只有当我包含不同的表时才会出现此错误？

Answer 1

对于诸如此类的复杂查询，一种相当便宜且也使查询更具可读性的技术是使用内联 View ，如下所示:

select Year, Sum(myValue) as Total
from (
    select 
    Case 
        When @Col = 1 Then YEAR(tbl1.myDate1)
        When @Col = 2 Then YEAR(tbl1.myDate2)
        When @Col = 3 Then YEAR(tbl2.myDate)
    End as [Year],
    tbl1.myValue
    From tabel1 as tbl1
    Left Join tabel2 as tbl2
    On tbl1.tbl2ID = tbl2.ID
)
Group By Year

注意双表 case 语句是如何在内联 View 中完成的，然后分组是作为第二遍完成的。需要注意的是，第二遍不执行任何额外的 I/O，这就是为什么使用这种技术相对便宜(而且方便)。

祝你好运!