How to COUNT with condition while distinct join MySQL? - mysql

Question

I have two tables:
mysql> DESCRIBE swaps;
+-------------+----------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+----------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| user1_id | int(11) | NO | | NULL | |
| user2_id | int(11) | NO | | NULL | |
| hasto | int(11) | NO | | NULL | |
| requested | datetime | NO | | NULL | |
| accepted | datetime | YES | | NULL | |
| swapped1 | datetime | YES | | NULL | |
| swapped2 | datetime | YES | | NULL | |
| rejected | datetime | YES | | NULL | |
| rejected_by | int(11) | YES | | NULL | |
+-------------+----------+------+-----+---------+----------------+
mysql> DESCRIBE messages;
+-----------+----------+------+-----+-------------------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------+----------+------+-----+-------------------+----------------+
| msg_id | int(11) | NO | PRI | NULL | auto_increment |
| sender_id | int(11) | NO | | NULL | |
| msg | text | NO | | NULL | |
| msg_time | datetime | NO | | CURRENT_TIMESTAMP | |
| swap_id | int(11) | NO | | NULL | |
| seen | datetime | YES | | NULL | |
+-----------+----------+------+-----+-------------------+----------------+
and query that I adjusted from this question
SELECT s.*, m.*
FROM swaps as s
JOIN messages as m
ON (s.id= m.swap_id AND m.msg_time=
(SELECT MAX(msg_time) FROM messages WHERE messages.swap_id = s.id));
as a result I single row for every swap and information about last sent message within this swap. I want to add the count of messages that have not jet been seen (m.seen IS NULL).
I tried different approaches but always get error. What I want is to add count of messages in corresponding swap with seen IS NULL to my result set. Would appreciate any suggestions.

Answer 1

You can add the count as a subquery:
SELECT s.*, m.*,
(SELECT COUNT(*) FROM messages m2 WHERE m2.seen IS NULL) as seen_is_null
FROM swaps s JOIN
messages m
ON s.id= m.swap_id AND
m.msg_time =(SELECT MAX(m2.msg_time) FROM messages m2 WHERE m2.swap_id = s.id));
It seems curious to be counting over all messages, but that is what the question asks for. You can, of course, introduce a correlation clause to count for a given swap or something else.