将连接计数与另一个连接列进行比较

Question

我想在

中想出我认为应该是一个非常简单的查询

相关机型：

一个Schedulehas_manyEvents，一个Eventhas_manyattendees。 我想获取所有 events 的 attendees 少于 event 的 schedule 的 min_attendees 列指定的内容.

SELECT "events".* FROM "events"
INNER JOIN "schedules" ON "schedules"."id" = "events"."schedule_id"
INNER JOIN "attendees" ON "attendees"."event_id" = "events"."id"
GROUP BY events.id
HAVING COUNT(attendees.id) < schedules.min_attendee

尝试上述查询时出现以下错误： ERROR: column "schedules.min_attendees" must appear in the GROUP BY clause or be used in an aggregate function

Answer 1

您可以通过在其上使用聚合函数来解决此问题：

SELECT "events".*
FROM "events" INNER JOIN
     "schedules"
      ON "schedules"."id" = "events"."schedule_id" INNER JOIN
      "attendees"
      ON "attendees"."event_id" = "events"."id"
GROUP BY events.id
HAVING COUNT(attendees.id) < MIN(schedules.min_attendee)

或者，将其包含在 group by:

SELECT "events".*
FROM "events" INNER JOIN
     "schedules"
      ON "schedules"."id" = "events"."schedule_id" INNER JOIN
      "attendees"
      ON "attendees"."event_id" = "events"."id"
GROUP BY events.id, schedules.min_attendee
HAVING COUNT(attendees.id) < schedules.min_attendee

Answer 2

如果您不将聚合函数放在 schedules.min_attendee 附近，那么它应该存在于 group by else 中，否则不需要。

这可能对你有帮助

SELECT "events".* FROM "events"
INNER JOIN "schedules" ON "schedules"."id" = "events"."schedule_id"
INNER JOIN "attendees" ON "attendees"."event_id" = "events"."id"
GROUP BY events.id
HAVING COUNT(attendees.id) < MAX(schedules.min_attendee)

您不能使用 MAX 或 MIN，因为每一行都会有所不同。无论您使用 SELECT 的什么列都必须在 GROUP BY 子句中，否则它会抛出错误（它必须出现在 group by 子句或 agg f'n 中）

将连接计数与另一个连接列进行比较

Compare join count to another joined column

sql

group-by

ruby-on-rails

having