我可以在select语句中使用别名吗？

Question

我想写一个看起来像这样的声明

select 1 as one, 2 as two , one + two as three

但是，SQL Server无法确定one + two是什么。

我可以用这种形式重写

SELECT x.one
      ,x.two
      ,x.one + x.two AS three
FROM (
    SELECT 1 AS one
          ,2 AS two
    ) x

这给了我期望的输出。它只是有点混乱（在一个非人为的例子中更是如此）我也不确定这类事情对SQL Server的内部做了什么以及对执行速度有何影响。

有更好的方法吗？

Answer 1

您无法在同一SELECT中引用别名，您需要在子查询中定义（如您所做）或在Common-table-expression(CTE)中定义：

WITH CTE AS
(
    SELECT 1 as one, 2 as two
)
SELECT one, two, one + two AS three FROM CTE

或者使用以下语法：

WITH CTE(one, two) AS
(
    SELECT 1, 2
)
SELECT one, two, one + two as three from CTE

同样的规则适用于WHERE：Reference alias (calculated in SELECT) in WHERE clause

但是，如果多次使用相同的表达式通常不会造成伤害，sql server优化器只会对它进行一次评估。所以你可以这样做：

SELECT 1 as one, 2 as two , 1 + 2 as three

Answer 2

您可以非常简单地命名您的值：

SELECT 
  one, two, one + two as three
FROM (values(1,2)) x(one,two)