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

WHERE version of @Keeper's solution

SELECT DISTINCT t1.post_id 
FROM tags t1, tags t2
WHERE 
  t1.post_id = t2.post_id  AND 
  t1.tag_id = 1 AND t2.tag_id = 3