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

try this. use group by with the name of the category. i have commented out the created at clause as you specified, you can uncomment it if you want to use it.

 SELECT c.name, SUM(ABS(v.item_id)) 
 FROM categories c,items i, votes v
 WHERE c.name = i.name
    AND i.item_id=v.item_id
    --AND v.created_at > #{1.week.ago}
 GROUP BY c.name 
 ORDER BY SUM(ABS(v.item_id)) DESC LIMIT 5;

you will notice that i did not use the JOIN keyword but instead filtered the results of the query using only WHERE clauses, which might be easier to understand. if you want to learn more about JOINs, here is a tutorial.

Here, too, is a tutorial on SQL aliases (the AS clause). in fact, there are a bunch more tutorials on this site for a bunch of different SQL topics that are not platform dependent.

edit: fixed as per comments, added the abs function,