Question

我有下表，小组成员数：

CREATE TABLE "group_members_count" (
  group_id INTEGER,
  time     TIMESTAMP,
  members_count INTEGER
)

我想查询具有自定义期间的特定组和时间间隔（从，到）的成员计数。

我写了以下查询：

SELECT 
  "group_id",
  date_trunc('day', "time") as period,
  max(count) as value
FROM
  "group_members_count"
WHERE
  "channel_id" IN (123, 124) AND ("time"::date >= '2017-11-02' AND "time"::date <= '2017-11-02')
GROUP BY 
  "period", "group_id"
ORDER BY
  "group_id", "period"

正如预期的那样，我每天都会获得最大值，但我需要最新。

Answer 1

一种方法是使用array_agg()并获取第一个元素：

SELECT group_id,
       date_trunc('day', "time") as period,
       max(count) as value,
       array_agg(count order by "time" desc)[1]
FROM group_members_count
WHERE channel_id IN (123, 124) AND
      ("time"::date >= '2017-11-02' AND "time"::date <= '2017-11-02')
GROUP BY period, group_id
ORDER BY group_id, period

查询PostgreSQL中的时间序列

1 个答案: