Question

假设有这个查询：

SELECT ((ColumnA*ColumnB)/ColumnC) AS ColDerA
       ,(ColumnD + ColumnE - ColumnF) AS ColDerB
       ,(ColDerA - ColDerB) AS ColDerC
FROM TableA

当我写(ColDerA - ColDerB) AS ColDerC SQL时返回此错误：
Invalid column name 'ColDerA'（ColDerB相同）如何创建ColDerC列？

感谢。

Answer 1

使用嵌套查询

SELECT ColDerA, ColDerB, (ColDerA - ColDerB) AS ColDerC
FROM
(
   SELECT ((ColumnA*ColumnB)/ColumnC) AS ColDerA
        ,(ColumnD + ColumnE - ColumnF) AS ColDerB
  FROM TableA
) t

Answer 2

原因是SQL从上到下（当你阅读代码时）不能以普通方式工作，因此对于用户而言，在用别名命名列之后可能很明显，别名可以在同一个中使用查询。不幸的是，SQL首先以批处理方式执行查询，因此为了使用查询返回的任何结果，您需要将其包装在子查询（嵌套查询）中。因此，您必须像这样使用它：

SELECT ColDerA, ColDerB, ColDerA - ColDerB FROM (
SELECT ((ColumnA*ColumnB)/ColumnC) AS ColDerA
   ,(ColumnD + ColumnE - ColumnF) AS ColDerB
   --,(ColDerA - ColDerB) AS ColDerC --this would work, if you had used directly columns from the table: ((ColumnA*ColumnB)/ColumnC) - (ColumnD + ColumnE - ColumnF)
FROM TableA ) AS A

T-SQL列由两列派生

2 个答案: