Question

一个初学者的总问题：我想使用GROUP BY语句运行子查询，然后在结果中找出具有最大值的行。我已经建立了如下表达式：

SELECT agg.facid, agg.Slots
FROM
(SELECT facid AS facid, SUM(slots) AS Slots FROM cd.bookings
GROUP BY facid
ORDER BY SUM(slots) DESC) AS agg
WHERE agg.Slots = (SELECT MAX(Slots) FROM agg);

在我看来，这应该首先创建一个带有facid和SUM（slots）值的2列表，然后将这些列分别命名为agg.facid和agg.Slots，我应该只在“插槽”。但是，相反，我收到此错误：

ERROR:  relation "agg" does not exist
LINE 6: WHERE agg.Slots = (SELECT MAX(Slots) FROM agg);

这可能很简单，所以对于一个愚蠢的问题，我先感到抱歉；）

我正在使用pgAdmin 4在PostgreSQL 10上工作。

Answer 1

使用Common Table Expression:

WITH agg AS (
    SELECT facid AS facid, SUM(slots) AS Slots 
    FROM cd.bookings
    GROUP BY facid
    )
SELECT agg.facid, agg.Slots
FROM agg
WHERE agg.Slots = (SELECT MAX(Slots) FROM agg);

Answer 2

这是您想要的吗？

SELECT
    facid,
    SUM(slots)
FROM cd.bookings
GROUP BY
    facid
HAVING SUM(slots) = MAX(slots)

Answer 3

因此，需要进行更多的研究，我发现了一个使用通用表表达式的解决方案，它可能足以满足我的喜好：

WITH sum AS (SELECT facid, SUM(slots) AS Slots FROM cd.bookings GROUP BY facid)
SELECT facid, Slots
FROM sum
WHERE Slots = (SELECT MAX(Slots) FROM sum);

第一行声明一个CTE，稍后可为子查询调用CTE，以计算聚合插槽列中的最大值是多少。希望它可以帮助任何有兴趣的人。

PostgreSQL：子查询上的聚合表达式

3 个答案: