在数据库中选择5个最新的唯一条目

Question

Struggling with an SQL query to select the 5 most recent, unique, entries in a MySQL 5.7.22 table. For example, here's the 'activity' table:

uaid    nid     created 
9222    29722   2018-05-17 03:19:33
9221    31412   2018-05-17 03:19:19
9220    31160   2018-05-16 23:47:34
9219    31160   2018-05-16 23:47:30
9218    31020   2018-05-16 22:35:59
9217    31020   2018-05-16 22:35:54
9216    28942   2018-05-16 22:35:20
...

The desired query should return the 5 most recent, unique entries by the 'nid' attribute, in this order (but only need the nid attribute):

uaid    nid     created 
9222    29722   2018-05-17 03:19:33
9221    31412   2018-05-17 03:19:19
9220    31160   2018-05-16 23:47:34
9218    31020   2018-05-16 22:35:59
9216    28942   2018-05-16 22:35:20

I have tried a variety of combinations of DISTINCT but none work, ie:

select distinct nid from activity order by created desc limit 5 

What is the proper query to return the 5 most recent, uniq entries by nid?

Answer 1

Your problem is the simplest form of the top-N-per-group problem. In general, this problem is a real headache to handle in MySQL, which doesn't support analytic functions (at least not in most versions folks are using in production these days). However, since you only want the first record per group, we can do a join to subquery which finds the max created value for each nid group.

SELECT a1.*
FROM activity a1
INNER JOIN
(
    SELECT nid, MAX(created) AS max_created
    FROM activity
    GROUP BY nid
) a2
    ON a1.nid = a2.nid AND a1.created = a2.max_created;

Demo

Answer 2

You can use a subquery and join

select * from activity m
inner join (
  select nid, min(created) min_date
  from activity 
  group by nid 
  limit 5
) t on t.nid = m.nin and t.min_date = m.created