如何使用 sub select 的值

Question

我有一个select:

select
b.[FiscalPeriod],
b.[Column2],b.[Current_Value],
(
    select TOP 1 [Column3] from [table1] b2 
    where month(b.ReportingPeriod) = month(DATEADD(month,1,b2.ReportingPeriod))
      AND YEAR(b.ReportingPeriod) = YEAR(DATEADD(month,1,b2.ReportingPeriod))
      AND b.id = b2.id
) as PREV_VALUE 
FROM [table1] b

现在我在做：(b.[Current_Value]-PREV_VALUE) as difference

但我收到错误：

Invalid column name 'PREV_VALUE'

我知道我可以再次放置 sub select 而不是 PREV_VALUE。但是如何避免重复select?

Answer 1

您无法在定义别名的地方访问 table 别名。在你的情况下，最好的解决方案可能是 outer apply:

select b.[FiscalPeriod], b.[Column2],b.[Current_Value], bb.PREV_VALUE
FROM [table1] b OUTER APPLY
     (select TOP 1 [Column3] as PREV_VALUE
      from [table1] b2 
      where month(b.ReportingPeriod) = month(DATEADD(month,1,b2.ReportingPeriod)) AND
            YEAR(b.ReportingPeriod) = YEAR(DATEADD(month,1,b2.ReportingPeriod)) AND
            b.id = b2.id
      order by ???
     ) bb

那么您可以在SELECT.

中多次访问该值

注意：使用TOP时应该使用ORDER BY，所以要填写???.

Answer 2

您可以将查询转换为派生的 table 或 CTE。然后你可以像列一样对待别名：

SELECT *, (Current_Value-PREV_VALUE) AS difference
FROM (
  Your current query
) q

如何使用 sub select 的值

How to use value from sub select

sql

sql-server

subquery

repeat

correlated-subquery