请教一个关联查询的 sql

This topic created in 1196 days ago, the information mentioned may be changed or developed.

数据库用的 postgres 14

现在有 3 张表, app, appuser, x ，其中 appuser 为 app 和 user 的映射关系，一个 app 会有多个 user

其中 app 的示例

id	name	x_id
1	1	1
2	1	2

appuser 示例

id	app_id	user_id
1	1	1
2	1	2

x 的示例

id	name
1	xxx
2	yyy

现在业务需求是做一个 app 的页面，其中一个表格，每行显示对应的 app.id, app.name, userCount, x.id, x.name

其中 x.id 和 x.name 把 app 表和 x 表 join 起来查询就可以。。。但是请教高手，可以怎样一行 sql 把该 app 对应的用户数也 count 出来？

6 replies • 2023-02-26 23:23:37 +08:00

F281M6Dh8DXpD1g2

Feb 26, 2023

理解一下 join 到底干了啥:
把多行满足条件的数据拼接到一行里面
现在你有这一行数据了,group by 不就完了

pendulum

Feb 26, 2023

SELECT "a"."id", "a"."name", COUNT("b"."user_id") AS "userCount", "x"."id", "x"."name" FROM "app" AS "a"
INNER JOIN "x" ON "a"."x_id" = "x"."id"
INNER JOIN "appuser" AS "b" ON "a"."id" = "b"."app_id"
GROUP BY "b"."app_id"

dreamramon

Feb 26, 2023

@pendulum #2
老是要提示这个。。。
> ERROR: column "app.id" must appear in the GROUP BY clause or be used in an aggregate function
高手帮忙看看呗。。。是不是这个 group by 有什么讲究～～～

mywind

Feb 26, 2023

SELECT app.id, app.name, COUNT(appuser.id) as userCount, x.id, x.name
FROM app
JOIN x ON app.x_id = x.id
LEFT JOIN appuser ON app.id = appuser.app_id
GROUP BY app.id, x.id;
试试这个

awen233333

Feb 26, 2023

@dreamramon select 里没有使用聚合函数的列都要放在 group by 语句后面

xiaoxixi

Feb 26, 2023

SELECT
app.id
, app.name
, COUNT(1) over(partition by appuser.app_id ) as userCount
, x.id
, x.name
FROM app
LEFT JOIN appuser ON app.id = appuser.app_id
LEFT JOIN x ON app.x_id = x.id