Question

The problem is very similar to the stack exchange voting system. I’d like to query an object table for a random object and also get back any votes a specified user has on that object.

objects table:
 id
 user_id
 message

votes table:
 object_id
 user_id
 vote_value

select * from objects order by RAND() produces a random object, but how can I select a specific user’s vote value for that object in the same query? It’s possible that the user never votes on the result object so I can’t simply do a join on id/object_id.

Answer 1

This query will print NULL for user_id and vote_value if the user (in this case 1) didn’t vote on an object:

SELECT o.id, v.user_id, v.vote_value 
FROM objects o LEFT OUTER JOIN votes v ON o.id = v.object_id AND v.user_id = 1
WHERE v.user_id = 1 OR v.user_id IS NULL
ORDER BY RAND() LIMIT 1;