Joining 3 Tables based on specific conditions - mysql

Question

I have the following 3 tables:
users: [id, name, admin ...]
events: [id, user_id, type ...]
messages: [id, user_id, ...]
I want to construct a query that does the following:
-> Select all users from the table users who have not scheduled an event of the type "collection"
-> And who have less than 3 messages of the type "collection_reminder"
-> And who are not admin
I've managed to figure out the first part of this query, but it all goes a bit pear shaped when I try to add the 3 table, do the count, etc.

Answer 1

Here is a query that might get the job done. Each of the requirement is represented as a condition in the WHERE clause, using correlated subqueries when needed:
SELECT u.*
FROM users u
WHERE
NOT EXISTS (
SELECT 1
FROM events e
WHERE e.user_id = u.id AND e.type = 'collection'
)
AND (
SELECT COUNT(*)
FROM messages m
WHERE m.userid = u.id AND m.type = 'collection_reminder'
) <= 3
AND u.admin IS NULL

Answer 2

Ill try this on the top of the head so expect some synthax issues, but the idea is the following.
You can filter out who have no events schedule using a left join. On a left join the elements on the second part of the query will show up as null.
select * from users u
left join events e on e.user_id = u.id
where e.user_id is null
Now, i dont think this is the most performant way, but a simple way to search for everyone that has 3 or less messages:
select * from users u
left join events e on e.user_id = u.id
where u.id in (
select COUNT(*) from messages m where m.user_id = u.id HAVING COUNT(*)>3;
)
and e.user_id is null
Then filtering who is not admin is the easiest :D
select * from users u
left join events e on e.user_id = u.id
where u.id in (
select COUNT(*) from messages m where m.user_id = u.id HAVING COUNT(*)>3;
)
and e.user_id is null
and u.admin = false
Hope it helps.

Answer 3

This is pretty much a direct translation of your requirements, in the order you listed them:
SELECT u.*
FROM users AS u
WHERE u.user_id NOT IN (SELECT user_id FROM events WHERE event_type = 'Collection')
AND u.user_id IN (
SELECT user_id
FROM messages
WHERE msg_type = 'Collection Reminder'
GROUP BY user_id
HAVING COUNT(*) < 3
)
AND u.admin = 0
or alternatively, this can be accomplished completely with joins:
SELECT u.*
FROM users AS u
LEFT JOIN events AS e ON u.user_id = e.user_id AND e.event_type = 'Collection'
LEFT JOIN messages AS m ON u.user_id = m.user_id AND m.msg_type = 'Collection Reminder'
WHERE u.admin = 0
AND e.event_id IS NULL -- No event of type collection
GROUP BY u.user_id -- Note: you should group on all selected fields, and
-- some configuration of MySQL will require you do so.
HAVING COUNT(DISTINCT m.message_id) < 3 -- Less than 3 collection reminder messages
-- distinct is optional, but
-- if you were to remove the "no event" condition,
-- multiple events could multiply the message count.
;

Answer 4

This query uses joins to link the 3 tables, filters the result using the where clause, and using Group by, having limiting the result to only those who satisfy the less than count condition..
SELECT a.id,
SUM(CASE WHEN b.type = 'collection' THEN 1 ELSE 0 END),
SUM(CASE WHEN c.type = 'collection_reminder' THEN 1 ELSE 0 END
FROM users a
left join events b on (b.user_id = a.id)
left join messages c on (c.user_id = a.id)
WHERE a.admin = false
GROUP BY a.id
HAVING SUM(CASE WHEN b.type = 'collection' THEN 1 ELSE 0 END) = 0
AND SUM(CASE WHEN c.type = 'collection_reminder' THEN 1 ELSE 0 END) < 3