Question

I am trying to create select statement in procedure.

Here is my code:

SELECT u.id, max(a.time), max(b.time)
FROM buy a, sell b, users u
WHERE a.user_id = u.id AND b.user_id = u.id
GROUP BY u.id;

I need to take the last date between max(a.time) and max(b.time).

Thanks in advance

Answer 1

SELECT u.id, max(case when a.time > b.time then a.time else b.time end)
FROM buy a, sell b, users u
WHERE a.user_id = u.id AND b.user_id = u.id
GROUP BY u.id;

If your DB supports aliased queries, try this which would be much more efficient:

SELECT u.id, case when a_max > b_max then a_max else b_max end
FROM (SELECT u.id, max(a.time) as a_max, max(b.time) as b_max
    FROM buy a, sell b, users u
    WHERE a.user_id = u.id AND b.user_id = u.id
    GROUP BY u.id) x;

The inner query could use indexes if they existed to get the max for each column quickly.

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While we’re at it, let’s re-code it using the modern join syntax:

SELECT u.id, max(case when a.time > b.time then a.time else b.time end)
FROM buy a
JOIN sell b ON b.user_id = u.id
JOIN users u ON a.user_id = u.id
GROUP BY u.id;