使用别名列计算另一个别名列

Question

我有以下SQL语句：

SELECT SomeData.., Order.EurAmount AS OrderTotalEeur, 
--Begin Subselect
(SELECT Sum(Invoice.EurAmount) 
FROM Invoice WHERE Invoice.OrderId = Order.Id 
GROUP BY Invoice.OrderId) As InvoiceTotalEur
--End Subselect
([Order].EurAmount - IIF(InvoiceTotalEur is null, 0, InvoiceTotalEur) AS Diff
FROM [Order]

正如你所看到的，我希望获得2笔金额（OrderTotalEur和InvoiceTotalEur），没什么大不了的。但是我也想使用这两个别名来计算另一个不起作用的列（Diff），因为它不能识别用于计算diff列的两个别名。

我可以做的是重复Subselect两次（对于iif和值）但这不是正确的方法，对吗？

Answer 1

您可以将查询视为子查询：

  SELECT SomeData..,  
    OrderTotalEeur,
    InvoiceTotalEur,
    (OrderTotalEeur - IIF(InvoiceTotalEur is null, 0, InvoiceTotalEur) AS Diff
  FROM (
      SELECT SomeData.., Order.EurAmount AS OrderTotalEeur, 
      --Begin Subselect
      (SELECT Sum(Invoice.EurAmount) 
      FROM Invoice WHERE Invoice.OrderId = Order.Id 
       GROUP BY Invoice.OrderId) As InvoiceTotalEur
      --End Subselect
    FROM [Order]
    ) myquery

然后在上一级你可以访问别名。