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

I've made some assumptions about your other tables. (i.e. that you have a table for posts that I have called posts and one with tag_id as the PK which I have called tag_table to avoid a nameclash with the posts/tags table that I can see you already call tags)

You want posts where there does not exist a tag in the list {1,3} for which there does not exist a matching record with the corresponding post_id/tag_id so you can use a double NOT EXISTS construct as below.

SELECT post_id
FROM posts p
WHERE NOT EXISTS 
    (SELECT * FROM tag_table tt
    WHERE tag_id IN (1,3)
    AND NOT EXISTS
        (SELECT * FROM tags t
        WHERE t.tag_id = tt.tag_id  and
        p.post_id = t.post_id)        
    )

Another alternative approach is to use Group By and Count. A review of approaches to this problem is here.