使用 GROUP_CONCAT 时，如何获取每个组的最新 2 条记录？

Question

SELECT id, GROUP_CONCAT(type SEPARATOR '/') as types FROM `sems` GROUP by year

I just want to get the 2 latest record on each group

Answer 1

If you are running MySQL 8.0, you can use row_number() for this. You need an ordering column to define the latest record, I assumed ordering_id :

select id, group_concat(type order by rn separator '/') types
from (
    select id, type, row_number() over(partition by id order by ordering_id desc) rn
    from sems
) t
where rn <= 2
group by id

In earlier versions, one option is to filter with a subquery:

select id, group_concat(type order by rn separator '/') types
from sems s
where s.ordering_id >= (
    select s1.ordering_id
    from sems s1
    where s1.id = s.id
    order by s1.ordering_id desc
    limit 2
)
group by id

This assumes that (id, ordering_id) tuples are unique. If not, and there are ties in the top 2, all related records will be taken into account.

Answer 2

If your data is not too numerous, you can use:

SELECT id,
       SUBSTRING_INDEX(GROUP_CONCAT(type ORDER BY ? DESC SEPARATOR '/'), '/', -2) as types
FROM `sems`
GROUP by year;.

The ? is for the column used to define "latest"