Select all projects that have matching tags

Question

I\'m trying to find the most efficient way of dealing with this but I must tell you front-head I\'ve made a mess of it. Looked around SO and found nothing of relevance so he

Answer 1

How about... (example for project 1)

SELECT p.num, p.title
FROM projects_to_tags pt1, projects_to_tags pt2, projects p
where pt1.project_id = 1 and 
      pt2.project_id != 1 and 
      pt1.tag_id = pt2.tag_id and 
      p.num = pt2.project_id 
group by pt2.project_id

And maybe add a separate index for tag_id in projects_to_tags so you can use it alone, instead of the composite. No more type ALL. (Table Scan) Replacing both 1 with 4 give also the desired results.