Mysql: how to select groups having certain values?

Question

Say there is such table:

mysql> SELECT * FROM tags;
+---------+--------+
| post_id | tag_id |
+---------+--------+
|       1 |      2 |
|       1 |      3         


        

Answer 1

You could try a self join (N tag_id -> N join) but probably it's not fast

SELECT t1.post_id 
FROM tags t1 INNER JOIN tags t2 ON t1.post_id = t2.post_id 
WHERE t1.tag_id = 1 AND t2.tag_id = 3