如何在聯合查詢中的另一個選擇中使用選擇的結果？

Question

我有一個聯合查詢，我想在聯合查詢的“左側”中使用選擇的結果，在聯合查詢“右側”的選擇語句中。 下面的查詢工作正常（至少在 postgres 上），但我運行 query1 2 次，一次作為 query1，再次作為 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

我想做這樣的事情，所以我不必運行 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

我收到此錯誤消息：

錯誤：關系“query1”不存在第 3 行：從 query1 中選擇 count(zz) 作為 zz

有沒有辦法重寫這個查詢，這樣 query1 只運行一次？

對 Llama 先生回復的小修改效果很好，看起來像這樣（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

您正在尋找公共表表達式。

它們允許您定義結果並在查詢中多次使用它。

在你的拳頭情況下：

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

當大多數數據庫支持group by和rollup時，為什么會做這樣的事情？ 看來你想要這樣的東西：

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

Answer 3

將 query1 結果放入臨時表

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

現在在第二個查詢中使用臨時表

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

Answer 4

您可能需要一個公共表表達式來重用 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