sql - 如何逐列选择用户的最大日期？

Question

我想获取userId和group by userId的最大日期

我现在不知道它是怎么做的 :c我的 table :

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 table :

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;