Sorted data in groups

Question

Dataset:

id  uid     activity    postid  
1   20      A           1
2   20      A           1
3   6       A           1
4   3       A           1
5   6       A           1
6   13      A           1
7   13      B           1
8   18      B           1
9   18      B           1
10  1       A           1

Current Results:

id  uid     uid_list        groupCount  activity    postid
9   18      18,13           3           B           1
1   20      1,13,6,3,20     7           A           1

Expected Results:

id  uid     uid_list        groupCount  activity    postid
9   18      18,13           3           B           1
10  1       1,13,6,3,20     7           A           1

The query I have:

SELECT
    id,
    uid,
    GROUP_CONCAT(DISTINCT uid ORDER BY id DESC) as uid_list,
    COUNT(*) as groupCount,
    activity,
    postid
FROM (
    SELECT *
    FROM `user_activities`
    ORDER BY id DESC) as tbl
GROUP BY
    activity, postid
ORDER BY
    id DESC

I want to group by activity and postid while having the result in descending order by id. And want to have the latest id and uid for every group. I don't understand why this query doesn't return the expected output.

Answer 1

From what I understand id value is increasing. To get the latest values you could use an aggregate function MAX().

Also, your inner query with ordering is unnecessary because the engine has to sort the resultset by id anyways when building result for GROUP_CONCAT().

To retrieve uid for a particular id column you need to self join to the same table.

SELECT
    a.id, b.uid, a.uid_list, a.groupcount, a.activity, a.postid
FROM (
    SELECT
        MAX(id) as id,
        GROUP_CONCAT(DISTINCT uid ORDER BY id DESC) as uid_list,
        COUNT(*) as groupCount,
        activity,
        postid
    FROM user_activities a
    GROUP BY
        activity, postid
    ) a
    INNER JOIN user_activities b ON a.id = b.id