Sql 如何在不重复代码的情况下获得x值

有可能这样做吗

create table t(a int, b int); insert into t values (1,1),(1,2),(1,3),(2,1),(2,2),(2,3),(3,1),(3,2),(3,3); select * from t; a | b ---------- 1 | 1 1 | 2 1 | 3 2 | 1 2 | 2 2 | 3 3 | 1 3 | 2 3 | 3 select max(case when a = 1 then b else 0 end) as q, max(case when b = 1 then a else 0 end) as c, ( max(case when a = 1 then b else 0 end) + max(case when b = 1 then a else 0 end) ) as x from t 不能使用在SELECT子句的同一级别上提供的别名

你有两个选择：

通过直接使用表达式 查询：

select max(case when a = 1 then b else 0 end) as q, max(case when b = 1 then a else 0 end) as c, (q + c) as x from t

select
  max(case when a = 1 then b else 0 end) as q,
  max(case when b = 1 then a else 0 end) as c,
  (max(case when a = 1 then b else 0 end) + max(case when b = 1 then a else 0 end)) as x
from t

通过在子查询中换行 查询：

select max(case when a = 1 then b else 0 end) as q, max(case when b = 1 then a else 0 end) as c, (q + c) as x from t

select
  max(case when a = 1 then b else 0 end) as q,
  max(case when b = 1 then a else 0 end) as c,
  (max(case when a = 1 then b else 0 end) + max(case when b = 1 then a else 0 end)) as x
from t

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不幸的是，你不能这么做

别名不能在创建它们的同一级别中使用

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我认为临时表是必要的。

同样在SQLServer2005+中，您可以使用CTE

SELECT  q, 
        c,
        q + c as x
FROM
(
  select
      max(case when a = 1 then b else 0 end) as q,
      max(case when b = 1 then a else 0 end) as c
    from t
) d

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1答案可能因SQL供应商而异。MySQL？神谕SQL Server？2我打赌一个临时表包含前两列，然后从中选择两列加上新列，可以完成itTNX 100x！子查询是我一直在寻找的。