How to make this Mysql query work?

Question

My query:

SELECT * 
FROM forum_topics 
WHERE cat_id IN(1,2,3,4,5,6,7) 
ORDER BY last_message DESC 
LIMIT 7

I want to get the biggest and only one value of each cat_id (7 values total). How to correct this query to make it work if it's even possible?

There is forum topics and each has value last_message and I want to get the latest topic's message's time. Hope it's clear.

Thank you

Answer 1

MySQL doesn't have analytical function support, which is what you're really after:

SELECT x.*
  FROM (SELECT ft.*,
               CASE 
                  WHEN @cat_id = ft.cat_id THEN @rownum := @rownum + 1
                  ELSE @rownum := 1
               END AS rank,
               @cat_id = ft.cat_id
          FROM FORUM_TOPICS ft
          JOIN (SELECT @rownum := 0, @cat_id := -1) r
      ORDER BY ft.cat_id, ft.last_message DESC)
 WHERE x.rank = 1

This will provide a computed column called "rank", where the most recent row based on the last_message column will have the value of "1" per cat_id value. The outer query only gets the records whose rank value is one...