MySQL LEFT JOIN using MAX & GROUP BY on joined table?

Question

I\'ve got two tables (members and activities) and I\'m trying to query the members with the latest activity for each member. I\'ve got it working with two queries (one to get t

Answer 1

The "latest per group" problem is extremely common in SQL. There are countless examples of solutions to this very problem on this site alone.

If your timestamps are uniqe per member activity:

SELECT
  m.id,
  m.name,
  a.code activity_code,
  a.timestamp activity_timestamp,
  a.description activity_description
FROM
  members m
  INNER JOIN activities a ON a.member_id = m.id
WHERE
  a.timestamp = (SELECT MAX(timestamp) FROM activities WHERE member_id = m.id)

alternatively, if your activity ID is increasing monotonically with time:

  ...
WHERE
  a.id = (SELECT MAX(id) FROM activities WHERE member_id = m.id)

You don't need to group. But the query will benefit from an index on activities over (member_id, timestamp) or (member_id, id), respectively.

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EDIT

To show any members who have not logged an activity, use a left join like this.

SELECT
  m.id,
  m.name,
  a.code activity_code,
  a.timestamp activity_timestamp,
  a.description activity_description
FROM
  members m
  LEFT JOIN activities a ON 
    a.member_id = m.id
    AND a.timestamp = (SELECT MAX(timestamp) FROM activities WHERE member_id = m.id)

Note that there is no WHERE clause. Semantically, WHERE is applied after the joins are done. So a WHERE clause would remove the rows that the LEFT JOIN added, effectively giving in the same result as the original INNER JOIN.

But if you apply the additional predicate right in the join condition, the LEFT JOIN will work as expected.