Question

Heya!, I have the below query:

SELECT t1.pm_id 
  FROM fb_user_pms AS t1, 
       fb_user_pm_replies AS t2 
 WHERE (t1.pm_id = '{$pm_id}' 
   AND t1.profile_author = '{$username}' 
    OR t1.pm_author = '{$username}' 
   AND t1.pm_id = t2.pm_id 
   AND t2.pm_author = '{$username}' 
   AND COUNT(t2.reply_id) > 0) 
   AND t1.deleted = 0

However, I’m getting a grouping error – my guess is its caused by the AND COUNT(t2.reply_id) > 0?

How can I rectify the above query to make it work.

Hope someone can help.

Cheers!

Answer 1

The error is because you can’t use an aggregate function (COUNT, MIN, MAX, AVG, etc) in the WHERE clause, without it being inside a subquery. Only the HAVING clause allows you to use aggregates without being wrapped in subqueries.

But checking for replies to be more than zero is not necessary on an INNER JOIN – that guarantees that there will be at least one reply associated to the fb_user_pms record. The JOIN also means that the information in t1 will be duplicated for every supported record in fb_user_pm_replies. IE: If a fb_user_pms record has three fb_user_pm_replies records related to it, you’ll see the fb_user_pms record in the result set three times.

The query you want to use is:

SELECT t1.pm_id 
  FROM fb_user_pms AS t1 
 WHERE t1.pm_id = '{$pm_id}' 
   AND '{$username}' IN (t1.profile_author, t1.pm_author)
   AND t1.deleted = 0
   AND EXISTS(SELECT NULL
                FROM fb_user_pm_replies AS t2 
               WHERE t2.pm_id = t1.pm_id
                 AND t2.pm_author = '{$username}')

The EXISTS clause returns true or false, based on the WHERE criteria. It also won’t duplicate t1 results.