Question

I have a table that links entries with associated tags with the following data in it:

entry_id |  tag_id
1        |  1
2        |  1
3        |  1
1        |  2
2        |  2

I am trying to write a query that returns only entries tagged with 1 AND 2, in this example entries 1 and 2 would be returned, while 3 would not, because it does not have both tags. The current query I am using works but I know can’t be right:

SELECT entry_id, GROUP_CONCAT(DISTINCT tag_id ORDER BY tag_id)
FROM tags
GROUP BY entry_id
HAVING GROUP_CONCAT(DISTINCT tag_id ORDER BY tag_id) LIKE "%1,2%";

Answer 1

If (entry_id, tag_id) is unique:

SELECT entry_id
FROM yourtable
WHERE tag_id IN (1, 2)
GROUP BY entry_id
HAVING COUNT(*) = 2

An alternative approach that doesn’t require uniqueness and can also be faster:

SELECT T1.entry_id
FROM yourtable T1
JOIN yourtable T2
ON T1.entry_id = T2.entry_id
AND T1.tag_id = 1
WHERE T2.tag_id = 2