Question

我试图在

中想出我认为应该是一个非常简单的查询

相关型号：

Schedule has_many Events和Event has_many attendees。 我希望得到的events attendees比event的{{1}} schedule列指定的min_attendees少。{ /强>

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：

中

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

如果你没有在scheduleles.min_attendee附近放置聚合函数，那么它应该存在于group中，否则不需要。

这可能有助于你

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)

您可以使用MAX或MIN，因为每行都会有所不同。无论您使用SELECT的哪一列必须在GROUP BY子句中，否则它将抛出错误（它必须出现在group by子句或agg f＆＃39; n）

将连接计数与另一个连接列进行比较

2 个答案: