如何逐列选择用户的最大日期？

Question

我想获取userId和以userId分组的最大日期

我现在不怎么做：c 我的桌子：

Id: 1    userId:1        activitydate:2019-02-15 08:03:11
Id: 2    userId:1        activitydate:0001-01-01 00:00:00
Id: 3    userId:2        activitydate:2019-02-15 07:38:30
Id: 4    userId:2        activitydate:2019-02-15 06:30:30
Id: 5    userId:3        activitydate:2019-03-01 09:17:04



select userid,maxvalue(activitydate)
from wearabledata
group by userid

实际结果：

我的桌子：

Id: 1    userId:1        activitydate:2019-02-15 08:03:11
Id: 3    userId:2        activitydate:2019-02-15 07:38:30
Id: 5    userId:3        activitydate:2019-03-01 09:17:04

Answer 1

DISTINCT ON关键字正是您需要的解决方案：

SELECT DISTINCT ON (userid)
       userid,activitydate
FROM   wearabledata
ORDER  BY userid, activitydate DESC;

Answer 2

SELECT
  userId,
  max(activitydate) as maxvalue
FROM
  wearabledata
GROUP BY
  userId;