在选择查询中重用Case语句计算

Question

select 
ROW_NUMBER()OVER(PARTITION BY [ConsLookUpID] ORDER BY [ConsLookUpID]) as [Write-off-Count]  
,CASE
WHEN ROW_NUMBER()OVER(PARTITION BY [ConsLookUpID] ORDER BY [ConsLookUpID])=1
THEN 
        CASE
            WHEN DATEPART(DAY,GETDATE())>15
                    THEN FORMAT(DATEADD(MONTH,0,GETDATE()),'yyyy-MM') 
            ELSE
                FORMAT(DATEADD(MONTH,-1,GETDATE()),'yyyy-MM')
        END
WHEN ROW_NUMBER()OVER(PARTITION BY [ConsLookUpID] ORDER BY [ConsLookUpID])=2
THEN 
        CASE
            WHEN DATEPART(DAY,GETDATE())>15
                THEN FORMAT(DATEADD(MONTH,-1,GETDATE()),'yyyy-MM') 
            ELSE
                FORMAT(DATEADD(MONTH,-2,GETDATE()),'yyyy-MM')
        END
WHEN ROW_NUMBER()OVER(PARTITION BY [ConsLookUpID] ORDER BY [ConsLookUpID])=3
THEN 
        CASE
            WHEN DATEPART(DAY,GETDATE())>15
                THEN FORMAT(DATEADD(MONTH,-2,GETDATE()),'yyyy-MM') 
            ELSE
                FORMAT(DATEADD(MONTH,-3,GETDATE()),'yyyy-MM')
        END
ELSE ''
        END AS [Write-off-Month]
,[RevLookUpId]+'_'+[Write-off-Month] as [Write-off-Key]
FROM ABC

我想要的结果是使用Writ-off-Key列中的case语句结果作为Writ-off-Month的值。 我可以再次使用 case 语句，会给我答案，但我不想这样做，因为它会增加处理时间。

尝试在不同的查询中执行 case 语句计算，然后使用 conslookUpId 进行内部连接，但 conslookupId 正在重复，因此即使我使用了内部连接，它也正在执行交叉连接。

在我看来，在同一个查询中可能无法实现，但会有某种方法，但无法弄清楚。如果可能的话，我也想只使用单个查询。

Answer 1

SQL 有时很奇怪，会让你重复自己的话。但是，在这种情况下，您可以通过嵌套代码（或将其拉出到 CTE）来显着改进。请注意，无论如何您都必须执行此操作才能使窗口功能按您的预期工作：

WITH PreCompute As (
    SELECT RevLookUpId
        , ROW_NUMBER()OVER(PARTITION BY [ConsLookUpID] ORDER BY [ConsLookUpID]) as [Write-off-Count] 
        , CASE WHEN DATEPART(DAY, GETDATE()) > 15 THEN 1 ELSE 0 END As DayOffset
    FROM ABC 
)

SELECT 
    [Write-off-Count]
    ,CASE WHEN [Write-off-Count] BETWEEN 1 AND 3
          THEN FORMAT(DATEADD(MONTH, DayOffset - [Write-off-Count], GETDATE()), 'yyyy-MM')
          ELSE '' END As [Write-off-Month]
    ,[RevLookUpId]
        +'_'
        +CASE WHEN [Write-off-Count] BETWEEN 1 AND 3
              THEN FORMAT(DATEADD(MONTH, DayOffset - [Write-off-Count], GETDATE()), 'yyyy-MM')
              ELSE '' END as [Write-off-Key]
FROM PreCompute