Mysql: how to select groups having certain values?

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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;

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