SQL Server：列别名访问

Question

我有一个如下所示的查询：

Select (numberLateOrders / numberOfOrders) as percentFailed
From Orders

现在我希望能够添加Where子句并访问列别名percentFailed，但是在查询完成之前似乎无法识别别名。这是真的和/或有解决方法吗？

谢谢！

Answer 1

是的，列WHERE子句中的列别名将无法访问/可用，因为SQL中执行的逻辑顺序是SELECT column alias之后的WHERE所以WHERE将无法识别它们。您必须再次在WHERE中添加计算/表达式，如下所示

Select (numberLateOrders / numberOfOrders) as percentFailed
From Orders
WHERE (numberLateOrders / numberOfOrders) > some_value

另一个解决方案（不是更好的解决方案）是使用外部查询，如

select percentFailed
from 
(
    Select (numberLateOrders / numberOfOrders) as percentFailed
    From Orders 
) tab
where percentFailed > some_value

Answer 2

只是添加到现有答案

或者使用子查询

select
  percentFailed
from (
  Select (numberLateOrders / numberOfOrders) as percentFailed
  From Orders
) t
where
  percentFailed > 0

Answer 3

在哪里看到别名，只需重复表达

Select (numberLateOrders / numberOfOrders) as percentFailed
From Orders
WHERE (numberLateOrders / numberOfOrders) >0