SQL query to select posts belonging to multiple categories

Question

I am writing a web application similar to a blogging software. There are three tables as below

Posts Table: Post_id,Post_Text
Post_Tags Table: Post_id,Tag_i         


        

Answer 1

This is relational division.

Use GROUP BY and COUNT or double NOT EXISTS.

An example of the first approach would be.

SELECT pt.Post_id, p.Post_Text
FROM Post_Tags pt
JOIN Posts p ON p.Post_id = pt.Post_id
WHERE pt.Tag_id IN (1,2,3)
GROUP BY pt.Post_id
HAVING COUNT(DISTINCT pt.Tag_id) = 3