Question

I’ve been working through a MySQL query and am wondering if there is a better way to do this (without correlated subqueries, etc…). The query in question is

SELECT 
   (SELECT COUNT(*) as num_visits FROM visit WHERE data_type='gallery'
    and gallery.id=visit.object_id) as num_visits, gallery.id
FROM gallery
    ORDER BY num_visits desc

The relavant parts of the table structure are

gallery
-------
id

visit
-----
id
data_type --> enum('gallery','etc')
object_id --> ties to various tables

Any help with this would be greatly appreciated.

Answer 1

   SELECT COUNT(v.object_id) as num_visits, g.id
     FROM gallery g
LEFT JOIN visit v ON g.id=v.object_id AND v.data_type='gallery'
 GROUP BY g.id
 ORDER BY num_visits DESC

But the better solution (from performance perspective) would be to have the amount of visits precalculated and stored right in gallery table (or in some other)