如何使用sub select中的值

I have one select:

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

Now I'm doing: (b.[Current_Value]-PREV_VALUE) as difference

现在我正在做：（b。[Current_Value] -PREV_VALUE）作为差异

but I got error:

但是我得到了错误：

Invalid column name 'PREV_VALUE'

列名称'PREV_VALUE'无效

I know that instead of PREV_VALUE, once again I can put sub select. But how to avoid repeat the select?

我知道，而不是PREV_VALUE，我可以再次选择sub。但是如何避免重复选择呢？

2 个解决方案

#1

You cannot access a table alias where it is defined. In your case, the best solution is probably outer apply:

您无法访问定义它的表别名。在您的情况下，最好的解决方案可能是外部适用：

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

Then you can access the value more than once in the SELECT.

然后，您可以在SELECT中多次访问该值。

Note: When using TOP you should be using ORDER BY, so you should fill in the ???.

注意：使用TOP时你应该使用ORDER BY，所以你应该填写???。

#2

You can turn your query into a derived table or CTE. Then you can treat the aliases like columns:

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

#1