SQL To Find Most Popular Category

Question

I have 3 tables in my DB (MySQL).

categories (name:string)
items (name:string, category_id:int)
votes (value:int, item_id:int, created_at:datetime)
         


        

Answer 1

This is what I ended up using (slightly modified version of @akf's answer). The other answers were great also, but this one seemed the simplest to me as a novice. I find it interesting that it doesn't use any joins? Would not have thought this was possible. I like how clean and simple it is.

SELECT c.*, SUM(ABS(v.vote)) AS score
FROM categories c,items i, votes v
  WHERE c.id = i.category_id
  AND i.id = v.item_id
  AND v.created_at > '#{1.week.ago}'
GROUP BY c.id
ORDER BY score DESC LIMIT 5;

(I forgot to mention in the question that each table has an 'id' column, which you can see being used here.)

Summing the absolute value of the vote value works in this case since there are positives and negatives as some posters correctly pointed out, but I was thinking it would be even simpler to just count the number of vote records in the database. I tried "COUNT(v) AS score" but this didn't work. If anyone knows how please post a comment. Thanks everyone!