Pull a random ID from a mysql table that a particular user has not seen before

Using mysql and PHP, I'd like to setup a query which pulls a problem from the problems table which a particular user has not seen yet.

I have two tables, 1) problems table and 2) records table.

The problems table has two fields: p_id, prob. This table contains a list of problems to display to the user.

The records table has 4 fields: record_id, user_id, p_id, num_seen. Anytime a user sees a particular problem the user's id gets recorded (user_id), the problem he/she saw gets recorded (p_id) and the number of times that user has seen it gets recorded (num_seen).

The query I'm trying to write would select a p_id from the problems table, under the condition that this p_id has not yet been vie开发者_开发问答wed by a particular user (user_id).

This seems like it should be simple but my attempts to use "WHERE problems.p_id != records.p_id AND user_id=$id" haven't worked. Any advice on the correct query or a better way to get the desired result would be awesome.

SELECT p.* 
FROM   problems p 
       LEFT JOIN records r 
         ON r.user_id = 100 
            AND r.p_id = p.p_id 
WHERE  p.p_id IS NULL 
       AND p.p_id >= Rand() * (SELECT Max(p_id) 
                               FROM   problems) 
LIMIT  1 

Where 100 is the current user_id.

you need indexes on

• (problems.p_id)
• (records.user_id,records.p_id)