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

If there aren't any unique constraints try:

SELECT post_id 
FROM tags 
WHERE tag_id = 1 OR tag_id = 3 
GROUP BY post_id 
HAVING count(DISTINCT tag_id) = 2;

Or use this HAVING clause, if trying to detect only two tag_id values:

HAVING MIN(tag_id) <> MAX(tag_id)

If post_id and tag_id both have an unique constraint, this should work too:

SELECT post_id 
FROM tags 
WHERE tag_id = 1 OR tag_id = 3 
GROUP BY post_id 
HAVING count(*) = 2;