如何在联合查询中使用另一个选择的结果？

Question

我有一个联合查询，我想在联合查询“左侧”的 select 语句中使用联合查询“右侧”的 select 的结果。下面的查询工作正常（至少在 postgres 上），但我运行 query1 两次，一次作为 query1，再次作为 SameAsQuery1。

select x as zz from (select 69 as x) as query1
union all
select count(zz) as zz from 
      (select x as zz from (select 69 as x) as sameAsQuery1) as query2

我想做这样的事情，这样我就不必运行 query1 两次，但它不起作用：

select x as zz from (select 69 as x) as query1
union all
select count(zz) as zz from query1

我收到此错误消息：

错误：关系“query1”不存在第 3 行：从 query1 选择 count(zz) 作为 zz

有没有办法重写这个查询，使 query1 只运行一次？

对 Llama 先生的回复进行小修改效果很好，看起来像这样（注意添加“as q2”）：

WITH
query1 AS
(
    SELECT x AS zz FROM (SELECT 69 AS x) as q2
)
SELECT zz FROM query1
UNION ALL
SELECT COUNT(zz) AS zz FROM query1

Answer 1

您正在寻找通用表表达式。

它们允许您定义结果并在查询中多次使用它。

在你的第一个案例中：

WITH
query1 AS
(
    SELECT x AS zz FROM (SELECT 69 AS x)
)
SELECT zz FROM query1
UNION ALL
SELECT COUNT(zz) AS zz FROM query1