Joins on the same table - mysql

Question

Is there a more efficent way to do this? I keep thinking I'm missing something. Thanks
SELECT DISTINCT eventId
FROM event_tags_map
WHERE tagId in (
SELECT tagId FROM event_tags_map WHERE eventId=114778
) ORDER BY RAND() LIMIT 5;
I'm hitting the same table twice and I'm wondering if I can get the same results faster.
Table structure:
mysql> describe event_tags_map;
+---------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+------------------+------+-----+---------+-------+
| eventId | int(10) unsigned | NO | PRI | NULL | |
| tagId | int(10) unsigned | NO | PRI | NULL | |
+---------+------------------+------+-----+---------+-------+
Indexes:
mysql> show index from event_tags_map;
+----------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| event_tags_map | 0 | PRIMARY | 1 | eventId | A | 302032 | NULL | NULL | | BTREE | | |
| event_tags_map | 0 | PRIMARY | 2 | tagId | A | 604065 | NULL | NULL | | BTREE | | |
+----------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.02 sec)

Answer 1

I looks like you will need to refer to the original table twice, one way or another.
I would recommend not using the IN condition, which is not very scalable and has various conter-intuitive behaviors.
My first option to use a correlated subquery with an EXISTS condition. This is usually the most efficient way to check that something exists...
SELECT DISTINCT eventId
FROM event_tags_map m
WHERE EXISTS (
SELECT 1 FROM event_tags_map m1 WHERE m1.eventId = 114778 AND m1.tagId = m.tagId
)
ORDER BY RAND() LIMIT 5;
An alternative option is to use a self-INNER JOIN:
SELECT DISTINCT eventId
FROM event_tags_map m
INNER JOIN event_tags_map m1 ON m1.eventId = 114778 AND m1.tagId = m.tagId
ORDER BY RAND() LIMIT 5;
Both solutions should be able to take advantage of a composite index on event_tags_map(eventId, tagId).