sql - 如何在联合查询的另一个选择中使用选择的结果？

Question

我有一个联合查询，我想在联合查询“右侧”的选择语句中使用联合查询“左侧”中的选择结果。下面的查询正常工作（至少在 postgres 上），但我运行 query1 2 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，所以我不必运行 query1 2 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 中选择 count(zz) 作为 zz

有没有办法重写这个查询，所以 query1 只运行一次？

对 Llama 先生的回应稍作修改，效果很好，看起来像这样（注意添加了“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

您正在寻找公用表表达式。

它们允许您定义结果并在查询中多次使用它。

在您的第一个案例中：

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

Answer 2

group by当大多数数据库支持和时，为什么要这样做rollup？看来你想要这样的东西：

select x, count(*) as cnt
from <whatever>
group by x with rollup;

Answer 3

将query1结果放入临时表

select x as zz from (select 69 as x) as query1 into temptable

现在在第二个查询中使用临时表

select zz from temptable
union all
select count(zz) as zz from temptable 

Answer 4

您可能需要一个公用表表达式来重用 SELECT：

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