MySQL SELECT most frequent by group

Question

How do I get the most frequently occurring category for each tag in MySQL? Ideally, I would want to simulate an aggregate function that would calculate the mode of a column.

Answer 1

(Edit: forgot DESC in ORDER BYs)

Easy to do with a LIMIT in the subquery. Does MySQL still have the no-LIMIT-in-subqueries restriction? Below example is using PostgreSQL.

=> select tag, (select category from stuff z where z.tag = s.tag group by tag, category order by count(*) DESC limit 1) AS category, (select count(*) from stuff z where z.tag = s.tag group by tag, category order by count(*) DESC limit 1) AS num_items from stuff s group by tag;
    tag     | category | num_items 
------------+----------+-----------
 ba         |        8 |         1
 automotive |        8 |         1
 bananatree |        8 |         4
 bath       |        9 |         1
 bamboo     |        8 |         9
(5 rows)

Third column is only necessary if you need the count.