Joining 3 Tables based on specific conditions

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