Question

I have 2 tables, one has the categories within it, and the other has the distinct items in it.

What I want to do is to select
the item_id and cat_id, and limit it to 5 result for each category, for example:

cat_id item_id
1      1
1      2
1      3
1      4
2      5
2      6
3      7

etc…
The closest query i’ve come up with

SELECT cat.cat_id, cat.cat_name, item_id, item_author, item_name, item_pic_big
FROM item_table ipt
JOIN cat_table cat ON ipt.cat_id = cat.cat_id
GROUP BY item_id
HAVING COUNT(*) < 5
ORDER BY cat.cat_id

That’s what I found on stackoverflow, however if I want to change the count to let’s say… 2, it gives me the same result, and if I change the group by to cat_id, it only gives me 1 result.

Any help would be appreciated

Answer 1

What you actually need is something like:

SELECT
cat.cat_id, cat.cat_name, item_id, item_author, item_name, item_pic_big
FROM item_table a
JOIN cat_table b ON a.cat_id = b.cat_id
WHERE a.item_id IN (SELECT item_id 
                    FROM item_table 
                    WHERE cat_id = b.cat_id 
                    LIMIT 5)
GROUP BY a.item_id
ORDER BY b.cat_id

Unfortunately, if you try to run this you will get this disappointing
error message:

This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'

So you’ll need a workaround. You can see a list of possible workarounds here:
http://www.artfulsoftware.com/infotree/queries.php#104

EDIT: The first solution there can be translated to your structure like this:
(I don’t have your tables, so there may be minor column names issues)

SELECT temp2.cat_id, temp2.item_id,
       temp2.cat_name, temp2.item_author, 
       temp2.item_name, temp2.item_pic_big
FROM
    (SELECT
          temp.cat_id,
          temp.item_id,
          temp.cat_name,
          temp.item_author,
          temp.item_name,
          temp.item_pic_big,
          IF( @prev <> temp.cat_id, @rownum := 1, @rownum := @rownum+1 ) AS rank,
          @prev := temp.cat_id
    FROM (SELECT
         a.item_id,
         b.cat_id,
         b.cat_name,
         a.item_author,
         a.item_name,
         a.item_pic_big
         FROM item_table a
         JOIN cat_table b ON a.cat_id = b.cat_id
         ORDER BY cat_id, item_id) AS temp
    JOIN (SELECT @rownum := NULL, @prev := 0) AS r
    ORDER BY temp.cat_id, temp.item_id) as temp2
WHERE temp2.rank <= 5
ORDER BY temp2.cat_id, temp2.item_id;