MySQL COUNT(*) not counting result rows - mysql

Question

Simplified schema of m:n relation implementing a subscription model:
CREATE TABLE c (
id INT(11) PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(32)
) ENGINE=MyISAM CHARACTER SET=UTF8;
CREATE TABLE t (
id INT(11) PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(32)
) ENGINE=MyISAM CHARACTER SET=UTF8;
CREATE TABLE c2t (
id INT(11) PRIMARY KEY AUTO_INCREMENT,
cid INT(11) NOT NULL,
tid INT(11) NOT NULL,
dateStart DATE NULL,
dateEnd DATE NULL
) ENGINE=MyISAM CHARACTER SET=UTF8;
INSERT INTO c (name) VALUES ('mike'),('carl'),('suzy');
INSERT INTO t (name) VALUES ('plan1'),('plan2'),('plan3'),('plan4');
INSERT INTO c2t (cid, tid, dateStart, dateEnd) VALUES
(1, 1, '2014-01-01', '2014-07-31'),
(1, 2, '2014-08-01', '2015-07-31'),
(1, 1, '2015-08-01', null),
(1, 3, '2015-09-01', null),
(2, 1, '2014-01-01', '2015-07-31'),
(2, 2, '2015-08-01', '2015-09-30'),
(2, 3, '2015-09-30', null),
(3, 1, '2014-01-01', '2014-12-31'),
(3, 2, '2014-01-01', '2014-12-31'),
(3, 3, '2015-01-01', '2015-10-31'),
(3, 4, '2015-01-01', '2015-10-31');
I've developed a query to find the c's who have active subscriptions of t's:
SELECT c.*
FROM c
LEFT JOIN c2t ON c.id = c2t.cid
AND NOW() BETWEEN COALESCE(dateStart, '0000-00-00')
AND COALESCE(dateEnd, DATE_ADD(NOW(), INTERVAL 1 DAY))
GROUP BY c2t.cid
HAVING COUNT(c2t.id) > 0;
Result as expected:
id name
1 mike
2 carl
The problem arises when I try to count the result rows. The query is almost identical, I've just dropped in a COUNT(*):
SELECT COUNT(*)
FROM c
LEFT JOIN c2t ON c.id = c2t.cid
AND NOW() BETWEEN COALESCE(dateStart, '0000-00-00')
AND COALESCE(dateEnd, DATE_ADD(NOW(), INTERVAL 1 DAY))
GROUP BY c2t.cid
HAVING COUNT(c2t.id) > 0;
Result:
`COUNT(*)`
2
1
Expected result would be a single row containing the number of rows found (2). I can only assume that the GROUP BY is interfering, but have no idea how to work around. Explanations are most welcome.

Answer 1

Wrap everything with subquery and use COUNT in outer query:
SELECT COUNT(*)
FROM (
SELECT c.*
FROM c
LEFT JOIN c2t ON c.id = c2t.cid
AND NOW() BETWEEN COALESCE(dateStart, '0000-00-00')
AND COALESCE(dateEnd, DATE_ADD(NOW(), INTERVAL 1 DAY))
GROUP BY c2t.cid
HAVING COUNT(c2t.id) > 0
) AS sub

Answer 2

If the only thing you want returned is the number of c's who have active subscriptions, then you can simplify your query like this:
SELECT COUNT(DISTINCT c.id) AS cnt
FROM c
INNER JOIN c2t ON c.id = c2t.cid
AND NOW() BETWEEN COALESCE(dateStart, '0000-00-00')
AND COALESCE(dateEnd, DATE_ADD(NOW(), INTERVAL 1 DAY))
So, INNER JOIN is used in place of LEFT JOIN: there is no need to return c's with no matches in c2t, since these are not going to have any active subscriptions.
Also, there is no need to GROUP BY: the query returns just one row with the number of c's.
Finally, DISTINCT must be used in COUNT so as to avoid counting duplicate c.id values more than once.