如何在选择（SQL Server）中使用变量？

Question

如果我想计算一列并在多于 1 列中使用结果，如何在不进行两次计算的情况下执行此操作？

例子：

SELECT LOWER(SUBSTRING([NAME], 4, 100)) + '@somedomain.com' as EMail
      ,hashbytes('SHA1', LOWER(SUBSTRING([NAME], 4, 100)) + '@somedomain.com') as HashedEmail
FROM sometable

如何避免在不使用多个选择的情况下写两次？

Answer 1

派生表

SELECT
    EMail,
    hashbytes('SHA1', EMail) AS HashedEmail
FROM
    (
    SELECT LOWER(SUBSTRING([NAME], 4, 100)) + '@somedomain.com' as EMail
    FROM sometable
   ) foo

或 CTE：

;WITH cEMail AS
(
    SELECT LOWER(SUBSTRING([NAME], 4, 100)) + '@somedomain.com' as EMail
    FROM sometable
)
SELECT
    EMail,
    hashbytes('SHA1', EMail) AS HashedEmail
FROM
    cEMail

但是，在这种情况下，我会考虑使用预先计算了哈希值的计算列