Selecting a count of rows having a max value - mysql

Question

Working example: http://sqlfiddle.com/#!9/80995/20
I have three tables, a user table, a user_group table, and a link table.
The link table contains the dates that users were added to user groups. I need a query that returns the count of users currently in each group. The most recent date determines the group that the user is currently in.
SELECT
user_groups.name,
COUNT(l.name) AS ct,
GROUP_CONCAT(l.`name` separator ", ") AS members
FROM user_groups
LEFT JOIN
(SELECT MAX(added), group_id, name FROM link LEFT JOIN users ON users.id = link.user_id GROUP BY user_id) l
ON l.group_id = user_groups.id
GROUP BY user_groups.id
My question is if the query I have written could be optimized, or written better.
Thanks!
Ben

Answer 1

You actual query is not giving you the answer you want; at least, as far as I understand your question. John actually joined group 2 on 2017-01-05, yet it appears on group 1 (that he joined on 2017-01-01) on your results. Note also you're missing one Group 4.
Using standard SQL, I think the next query is what you're looking for. The comments in the query should clarify what each part is doing:
SELECT
user_groups.name AS group_name,
COUNT(u.name) AS member_count,
group_concat(u.name separator ', ') AS members
FROM
user_groups
LEFT JOIN
(
SELECT * FROM
(-- For each user, find most recent date s/he got into a group
SELECT
user_id AS the_user_id, MAX(added) AS last_added
FROM
link
GROUP BY
the_user_id
) AS u_a
-- Join back to the link table, so that the `group_id` can be retrieved
JOIN link l2 ON l2.user_id = u_a.the_user_id AND l2.added = u_a.last_added
) AS most_recent_group ON most_recent_group.group_id = user_groups.id
-- And get the users...
LEFT JOIN users u ON u.id = most_recent_group.the_user_id
GROUP BY
user_groups.id, user_groups.name
ORDER BY
user_groups.name ;
This can be written in a more compact way in MySQL (abusing the fact that, in older versions of MySQL, it doesn't follow the SQL standard for the GROUP BY restrictions).
That's what you'll get:
group_name | member_count | members
:--------- | -----------: | :-------------
Group 1 | 2 | Mikie, Dominic
Group 2 | 2 | John, Paddy
Group 3 | 0 | null
Group 4 | 1 | Nellie
dbfiddle here
Note that this query can be simplified if you use a database with window functions (such as MariaDB 10.2). Then, you can use:
SELECT
user_groups.name AS group_name,
COUNT(u.name) AS member_count,
group_concat(u.name separator ', ') AS members
FROM
user_groups
LEFT JOIN
(
SELECT
user_id AS the_user_id,
last_value(group_id) OVER (PARTITION BY user_id ORDER BY added) AS group_id
FROM
link
GROUP BY
user_id
) AS most_recent_group ON most_recent_group.group_id = user_groups.id
-- And get the users...
LEFT JOIN users u ON u.id = most_recent_group.the_user_id
GROUP BY
user_groups.id, user_groups.name
ORDER BY
user_groups.name ;
dbfiddle here