如何避免在同一个选择中多次重复计算表达式? [英] How can I avoid repeating a calculated expression multiple times in the same select?

问题描述

如何在同一个选择中多次使用计算列而不重复表达式，也无需使用公共表表达式或复杂的子选择?

How can I use a calculated column multiple times in the same select without repeating the expression and without using common table expressions or complex subselects?

DECLARE @T TABLE ( NUM1 INT,NUM2 INT)
INSERT INTO @T VALUES (2,3);
INSERT INTO @T VALUES (5,7);
INSERT INTO @T VALUES(32,3);
INSERT INTO @T VALUES(6,8);

SELECT (NUM1+NUM2) [ADD], [ADD]*2, [ADD]/2,* FROM @T

有没有办法在 SQL Server 2005 中解决这个问题?

Is there any way to solve this in SQL Server 2005?

推荐答案

可以使用交叉应用

SELECT T2.[ADD],
       T2.[ADD]*2,
       T2.[ADD]/2
FROM @T AS T1
  CROSS APPLY (SELECT T1.NUM1+T1.NUM2) AS T2([ADD])

或 CTE

WITH C AS
(
  SELECT NUM1+NUM2 AS [ADD]
  FROM @T
)
SELECT [ADD],
       [ADD]*2,
       [ADD]/2
FROM C

或子查询(也称为派生表)

or a subquery (also known as a derived table)

SELECT T.[ADD],
       T.[ADD]*2,
       T.[ADD]/2
FROM (
       SELECT NUM1+NUM2 AS [ADD]
       FROM @T
     ) AS T

不能在声明的同一个字段列表中使用列别名.

It is not possible to use the column alias in the same field list as it is declared.

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