selecting top N rows for each group in a table

Question

I am facing a very common issue regarding \"Selecting top N rows for each group in a table\".

Consider a table with id, name, hair_colour, score columns

Answer 1

The way the algorithm comes up with the rank, is to count the number of rows in the cross-product with a score equal to or greater than the girl in question, in order to generate rank. Hence in the problem case you're talking about, Sarah's grid would look like

a.name | a.score | b.name  | b.score
-------+---------+---------+--------
Sarah  | 9       | Sarah   | 9
Sarah  | 9       | Deborah | 9

and similarly for Deborah, which is why both girls get a rank of 2 here.

The problem is that when there's a tie, all girls take the lowest value in the tied range due to this count, when you'd want them to take the highest value instead. I think a simple change can fix this:

Instead of a greater-than-or-equal comparison, use a strict greater-than comparison to count the number of girls who are strictly better. Then, add one to that and you have your rank (which will deal with ties as appropriate). So the inner select would be:

SELECT a.id, COUNT(*) + 1 AS ranknum
FROM girl AS a
  INNER JOIN girl AS b ON (a.hair = b.hair) AND (a.score < b.score)
GROUP BY a.id
HAVING COUNT(*) <= 3

Can anyone see any problems with this approach that have escaped my notice?