GROUP BY + HAVING ignore row - mysql

Question

Basically what I wanted is that I can select all the race records with record holder and best time. I looked up about similar queries and managed to find 3 queries that were faster than the rest.
The problem is it completely ignores the race the userid 2 owns the record of.
These are my tables, indexes, and some sample data:
CREATE TABLE `races` (
`raceid` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(20) NOT NULL,
PRIMARY KEY (`raceid`),
UNIQUE KEY `name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
CREATE TABLE `users` (
`userid` mediumint(8) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(20) NOT NULL,
PRIMARY KEY (`userid`),
UNIQUE KEY `name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
CREATE TABLE `race_times` (
`raceid` smallint(5) unsigned NOT NULL,
`userid` mediumint(8) unsigned NOT NULL,
`time` mediumint(8) unsigned NOT NULL,
PRIMARY KEY (`raceid`,`userid`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
INSERT INTO `races` (`raceid`, `name`) VALUES
(1, 'Doherty'),
(3, 'Easter Basin Naval S'),
(5, 'Flint County'),
(6, 'Fort Carson'),
(4, 'Glen Park'),
(2, 'Palomino Creek'),
(7, 'Tierra Robada');
INSERT INTO `users` (`userid`, `name`) VALUES
(1, 'Player 1'),
(2, 'Player 2');
INSERT INTO `race_times` (`raceid`, `userid`, `time`) VALUES
(1, 1, 51637),
(1, 2, 50000),
(2, 1, 148039),
(3, 1, 120516),
(3, 2, 124773),
(4, 1, 101109),
(6, 1, 89092),
(6, 2, 89557),
(7, 1, 77933),
(7, 2, 78038);
So if I run these 2 queries:
SELECT rt1.raceid, r.name, rt1.userid, p.name, rt1.time
FROM race_times rt1
LEFT JOIN users p ON (rt1.userid = p.userid)
JOIN races r ON (r.raceid = rt1.raceid)
WHERE rt1.time = (SELECT MIN(rt2.time) FROM race_times rt2 WHERE rt1.raceid = rt2.raceid)
GROUP BY r.name;
or..
SELECT rt1.*, r.name, p.name
FROM race_times rt1
LEFT JOIN users p ON p.userid = rt1.userid
JOIN races r ON r.raceid = rt1.raceid
WHERE EXISTS (SELECT NULL FROM race_times rt2 WHERE rt2.raceid = rt1.raceid
GROUP BY rt2.raceid HAVING MIN(rt2.time) >= rt1.time);
I receive correct results as shown below:
raceid | name | userid | name | time |
-------+----------------------+--------+----------+--------|
1 | Doherty | 2 | Player 2 | 50000 |
3 | Easter Basin Naval S | 1 | Player 1 | 120516 |
6 | Fort Carson | 1 | Player 1 | 89092 |
4 | Glen Park | 1 | Player 1 | 101109 |
2 | Palomino Creek | 1 | Player 1 | 148039 |
7 | Tierra Robada | 1 | Player 1 | 77933 |
and here is the faulty query:
SELECT rt.raceid, r.name, rt.userid, p.name, rt.time
FROM race_times rt
LEFT JOIN users p ON p.userid = rt.userid
JOIN races r ON r.raceid = rt.raceid
GROUP BY r.name
HAVING rt.time = MIN(rt.time);
and the result is this:
raceid | name | userid | name | time |
-------+----------------------+--------+----------+--------|
3 | Easter Basin Naval S | 1 | Player 1 | 120516 |
6 | Fort Carson | 1 | Player 1 | 89092 |
4 | Glen Park | 1 | Player 1 | 101109 |
2 | Palomino Creek | 1 | Player 1 | 148039 |
7 | Tierra Robada | 1 | Player 1 | 77933 |
As you can see, race "Doherty" (raceid: 1) is owned by "Player 2" (userid: 2) and it is not shown along with the rest of race records (which are all owned by userid 1). What is the problem?
Regards,

Answer 1

Having is a post filter. The query gets all the results, and then further filters them based on having. The GROUP BY compacting the rows based on the group, which gives you the first entry in each set. Since player 1 is the first entry for race 1, that's the result that is being processed by the HAVING. It is then filtered out because its time does not equal the MIN(time) for the group result.
This is why the other ones you posted are using a sub-query. My personal preference is for the first example, as to me it's slightly easier to read. Performance wise they should be the same.
While it's not a bad idea to try and avoid sub queries in the where clause, this is mostly valid when you can accomplish the same result with a JOIN. Other times it's not possible to get the result with a JOIN and a sub query is required.