Question

我有一个选择：

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

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

但我收到了错误：

列名称无效＆＃39; PREV_VALUE＆＃39;

我知道而不是PREV_VALUE，我可以再次选择。但是如何避免重复选择？

Answer 1

您无法访问定义它的表别名。在您的情况下，最佳解决方案可能是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

然后，您可以在SELECT。

中多次访问该值

注意：使用TOP时，您应该使用ORDER BY，因此您应该填写???。

Answer 2

您可以将查询转换为派生表或CTE。然后你可以像列一样处理别名：

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

如何使用sub select中的值

2 个答案: