Question

I have a table, named items.
The structure is:
{name, id, category_id}

There are unknown number of records in the table, I need to limit the number of records for each category to a MAX number, 20 for example.

Pseudo

DELETE FROM items
WHERE (SELECT COUNT(category_id) FROM items)> 20
LIMIT (SELECT COUNT(category_id) FROM items) - 20

Answer 1

DELETE FROM items 
WHERE (category_id,id) not in (
  SELECT category_id, id FROM items  a WHERE id IN (
     SELECT id FROM items s 
     WHERE a.category_id = s.category_id
     ORDER BY category_id LIMIT 20) 
  ORDER BY  id)