MYSQL: select the top N candidates - mysql

Question

I have the following DB scheme for university elections:
for each department, I have the following positions:
1 CHEF (which is the candidate_position = 1)
& 6 Members (which is the candidate_position = 2)
I want to obtain the winners of the election in each department.
to obtain the winner of CHEF position in "Informatique" department, I did the following query:
SELECT doctor.firstname, doctor.lastname, votes
FROM (SELECT COUNT(*) AS votes FROM candidate_votes WHERE candidate_votes.candidate_position = 1 GROUP BY candidate_votes.candidate_id) AS votes, doctor
INNER JOIN department_candidates ON department_candidates.doctor_id = doctor.id
INNER JOIN department ON department.id = department_candidates.department_id AND department.name = 'Informatique'
INNER JOIN candidate_votes ON candidate_votes.candidate_id = doctor.id AND candidate_votes.candidate_position = 1
GROUP BY candidates_votes.candidate_id
please note I didn't use LIMIT 1 because may be there is a tie (or draw) of votes between multiple candidates
based on the results, I think that my query of selecting the winner of Chef position is right, But I want some help to know how to select the first 6 candidates of Member position ?
Data set:
--
-- Table structure for table `candidate_votes`
--
DROP TABLE IF EXISTS `candidate_votes`;
CREATE TABLE IF NOT EXISTS `candidate_votes` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`candidate_id` int(11) NOT NULL,
`voter_id` int(11) NOT NULL,
`candidate_position` tinyint(1) NOT NULL COMMENT '1: chef, 2: member',
`date` date NOT NULL,
PRIMARY KEY (`id`),
KEY `fk-candidate_votes-voter_id` (`voter_id`),
KEY `fk-candidate_votes-candidate_id_idx` (`candidate_id`)
) ENGINE=InnoDB AUTO_INCREMENT=36 DEFAULT CHARSET=utf8;
--
-- Dumping data for table `candidate_votes`
--
INSERT INTO `candidate_votes` (`id`, `candidate_id`, `voter_id`, `candidate_position`, `date`) VALUES
(24, 2, 1, 1, '2018-05-26'),
(25, 1, 1, 2, '2018-05-26'),
(26, 6, 1, 2, '2018-05-26'),
(27, 5, 1, 2, '2018-05-26'),
(28, 7, 1, 2, '2018-05-26'),
(29, 8, 1, 2, '2018-05-26'),
(30, 9, 1, 2, '2018-05-26'),
(31, 2, 2, 1, '2018-05-16'),
(32, 3, 7, 1, '2018-05-22'),
(33, 3, 8, 1, '2018-05-22'),
(34, 4, 6, 2, '2018-05-29'),
(35, 7, 6, 2, '2018-05-29');
-- --------------------------------------------------------
--
-- Table structure for table `department`
--
DROP TABLE IF EXISTS `department`;
CREATE TABLE IF NOT EXISTS `department` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(255) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `department-name` (`name`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;
--
-- Dumping data for table `department`
--
INSERT INTO `department` (`id`, `name`) VALUES
(1, 'Informatique'),
(2, 'Mathematique'),
(4, 'physique');
-- --------------------------------------------------------
--
-- Table structure for table `department_candidates`
--
DROP TABLE IF EXISTS `department_candidates`;
CREATE TABLE IF NOT EXISTS `department_candidates` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`department_id` int(11) NOT NULL,
`doctor_id` int(11) NOT NULL,
`candidate_position` tinyint(1) NOT NULL COMMENT '1: chef, 2: member',
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=15 DEFAULT CHARSET=utf8;
--
-- Dumping data for table `department_candidates`
--
INSERT INTO `department_candidates` (`id`, `department_id`, `doctor_id`, `candidate_position`) VALUES
(5, 1, 3, 1),
(7, 1, 4, 2),
(8, 1, 1, 2),
(9, 1, 2, 1),
(10, 1, 6, 2),
(11, 1, 5, 2),
(12, 1, 7, 2),
(13, 1, 8, 2),
(14, 1, 9, 2);
-- --------------------------------------------------------
--
-- Table structure for table `doctor`
--
DROP TABLE IF EXISTS `doctor`;
CREATE TABLE IF NOT EXISTS `doctor` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`firstname` varchar(255) NOT NULL,
`lastname` varchar(255) NOT NULL,
`department_id` int(11) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8;
--
-- Dumping data for table `doctor`
--
INSERT INTO `doctor` (`id`, `firstname`, `lastname`, `department_id`) VALUES
(1, 'doc1_fn', 'doc1_ln', 1),
(2, 'doc2_fn', 'doc2_ln', 1),
(3, 'doc3_fn', 'doc3_ln', 1),
(4, 'doc4_fn', 'doc4_ln', 1),
(5, 'doc5_fn', 'doc5_ln', 1),
(6, 'doc6_fn', 'doc6_ln', 1),
(7, 'doc7_fn', 'doc7_ln', 1),
(8, 'doc8_fn', 'doc8_ln', 1),
(9, 'doc9_fn', 'doc9_ln', 1);
-- --------------------------------------------------------
Sqlfiddle DEMO

Answer 1

Consider the following:
SELECT x.*
, CASE WHEN #prev_position = candidate_position THEN CASE WHEN #prev_total = total THEN #i:=#i ELSE #i:=#i+1 END ELSE #i:=1 END i
, #prev_position := candidate_position prev_position
, #prev_total := total prev_total
FROM
(
SELECT candidate_id
, candidate_position
, COUNT(*) total
FROM candidate_votes
GROUP
BY candidate_id
, candidate_position
) x
JOIN
( SELECT #prev_position := null,#prev_total:=null,#i:=0) vars
ORDER
BY candidate_position
, total DESC;
+--------------+--------------------+-------+------+---------------+------------+
| candidate_id | candidate_position | total | i | prev_position | prev_total |
+--------------+--------------------+-------+------+---------------+------------+
| 2 | 1 | 2 | 1 | 1 | 2 |
| 3 | 1 | 2 | 1 | 1 | 2 |
| 7 | 2 | 2 | 1 | 2 | 2 |
| 8 | 2 | 1 | 2 | 2 | 1 |
| 9 | 2 | 1 | 2 | 2 | 1 |
| 1 | 2 | 1 | 2 | 2 | 1 |
| 4 | 2 | 1 | 2 | 2 | 1 |
| 5 | 2 | 1 | 2 | 2 | 1 |
| 6 | 2 | 1 | 2 | 2 | 1 |
+--------------+--------------------+-------+------+---------------+------------+
In this example, i represents rank. For position 1, we can see that two candidates tied for first place. For position 2, there was one outright winner, with all remaining candidates tying for second place.

Answer 2

Apparently I was trying to be too clever in my other answer, you can achieve a simple ranking table like this:
SELECT cast(dc.candidate_position AS UNSIGNED) AS position, dc.doctor_id, doc.firstname, doc.lastname, v.votes
FROM department_candidates dc
JOIN department dept ON dept.id=dc.department_id AND dept.name='Informatique'
JOIN doctor doc ON doc.id=dc.doctor_id
JOIN (SELECT candidate_position AS cp, candidate_id AS cid, count(candidate_id) AS votes
FROM candidate_votes
GROUP BY cid) v
ON v.cid=doc.id AND v.cp = dc.candidate_position
ORDER BY position, v.votes DESC
Output:
position doctor_id firstname lastname votes
1 3 doc3_fn doc3_ln 2
1 2 doc2_fn doc2_ln 2
2 7 doc7_fn doc7_ln 2
2 4 doc4_fn doc4_ln 1
2 1 doc1_fn doc1_ln 1
2 6 doc6_fn doc6_ln 1
2 5 doc5_fn doc5_ln 1
2 8 doc8_fn doc8_ln 1
2 9 doc9_fn doc9_ln 1
Demo

Answer 3

This query will give you the number of votes required to be a winner for a given position (note I have parameterised it with variables):
SET #position = 2;
SET #numwinners = 3;
SELECT #rank := #rank+1 AS rank, votes
FROM (SELECT COUNT(candidate_id) AS votes
FROM candidate_votes
WHERE candidate_position = #position
GROUP BY candidate_id
ORDER BY votes DESC) vr
JOIN (select #rank := 0) r
GROUP BY rank
HAVING rank = #numwinners
Output for your data with #position=2 and #numwinners=3:
rank votes
3 1
Having figured out how many votes are required to be a winner, we just need to find all candidates that have the required number of votes. Since the results are based on number of votes, ties are automatically taken care of. This query will generate that output:
SET #position = 2;
SET #numwinners = 3;
SELECT doc.firstname, doc.lastname, v.votes
FROM department_candidates dc
JOIN department dept ON dept.id=dc.department_id AND dept.name='Informatique'
JOIN doctor doc ON doc.id=dc.doctor_id
JOIN (SELECT candidate_id AS cid, count(candidate_id) AS votes
FROM candidate_votes
WHERE candidate_position = #position
GROUP BY cid) v
ON v.cid=dc.id
WHERE v.votes >= (SELECT votes
FROM (SELECT #rank := #rank+1 AS rank, votes
FROM (SELECT COUNT(candidate_id) AS votes
FROM candidate_votes
WHERE candidate_position = #position
GROUP BY candidate_id
ORDER BY votes DESC) vr
JOIN (select #rank := 0) r
GROUP BY rank
HAVING rank = #numwinners
) vt
)
ORDER BY v.votes DESC
Output for your data with #position=2 and #numwinners=3:
firstname lastname votes
doc7_fn doc7_ln 2
doc4_fn doc4_ln 1
doc1_fn doc1_ln 1
doc6_fn doc6_ln 1
doc5_fn doc5_ln 1
doc8_fn doc8_ln 1
doc9_fn doc9_ln 1
Output for your data with #position=1 and #numwinners=1:
firstname lastname votes
doc3_fn doc3_ln 2
doc2_fn doc2_ln 2
If you want to ensure the query works even if the value of #numwinners is greater than the number of candidates, change:
HAVING rank = #numwinners
to
HAVING rank = LEAST(#numwinners, (SELECT COUNT(DISTINCT candidate_id)
FROM candidate_votes
WHERE candidate_position = #position))
Demo