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

SELECT post_id
  FROM ( SELECT post_id,
                count(tag_id) AS counter
           FROM tags
          WHERE tag_id IN (1,3)
          GROUP BY post_id
       )
 WHERE counter = 2

Use GROUP_CONCAT() for the second part of your question

SELECT post_id,
       GROUP_CONCAT(tag_id ORDER BY tag_id ASC SEPARATOR ',')
  FROM tags