How to make a query with AND operator between rows [duplicate] - mysql

Question

This question already has answers here:
Select values that meet different conditions on different rows?
(6 answers)
Closed 3 years ago.
I have the following table, witch is the relationship pivot table, between posts and tags:
post_tags table
I need to get the posts that strict contain a 'X' given tag(s).
For example:
If I need posts with exclusively tags 1 and 2, it should returns post_id 1 and 4.
If I need post with tag 2, it should only returns post_id 3.
If I need post with tag 23, it should't returns nothing.
I've tried with:
SELECT * FROM `post_tags` WHERE tag_id = 1 OR tag_id = 2;
but obviously it returns all post_id with these tags_id
And with:
SELECT * FROM `post_tags` WHERE tag_id = 1 AND tag_id = 2;
It doest's return anything, because it's trying to comparate between columns.
Any solution?

Answer 1

You need to group by post_id and check the conditions in the having clause:
SELECT post_id
FROM post_tags
GROUP BY post_id
HAVING
SUM(tag_id NOT IN (1, 2)) = 0
AND
COUNT(DISTINCT tag_id) = 2
This will return only posts with tags 1 and 2 and no other tag.
For posts with only tag 2:
SELECT post_id
FROM post_tags
GROUP BY post_id
HAVING
SUM(tag_id <> 2) = 0
AND
COUNT(DISTINCT tag_id) = 1
If each post_id, tag_id pair is unique, then you can do this:
SELECT post_id
FROM post_tags
GROUP BY post_id
HAVING COUNT(tag_id IN (1, 2)) = COUNT(tag_id)

Answer 2

You could use a correlated subquery:
SELECT *
FROM post_tags pt
WHERE (
SELECT count(*)
FROM post_tags
WHERE pt.post_id = post_id
AND post_tag IN (1, 2)
) = 2
Or you could use an in list (using similar logic):
SELECT *
FROM post_tags pt
WHERE post_id IN (
SELECT post_id
FROM post_tags
WHERE post_tag IN (1, 2)
GROUP BY post_id
HAVING count(*) = 2
)