select the candidates with all the skills using GROUP_CONCAT - php

Question

I have the MySql tables for candidates, candidate-skills, and skills.
Which is the best way to select the candidates with all the skills
I tried using the following query. But it is not accurate.
Select `t`.*, GROUP_CONCAT(DISTINCT(s.name)) as skills,
GROUP_CONCAT(DISTINCT(s.id)) as skill_ids
FROM `candidates` `t`
LEFT JOIN `candidate-skills` `cs` ON `t`.`id` = `cs`.`can_id`
LEFT JOIN `skills` `s` ON `cs`.`skill_id` = `s`.`id`
where s.id in ('8','10')
GROUP BY `t`.`id`
ORDER BY `t`.`id` desc
The two points what I want are:
All the skills should be shown (when commenting the where conditions in the )
Records with all skills are to be shown. (The records with one skill is also showing as I am using the where in array)
I am using codeigniter framework.
http://sqlfiddle.com/#!9/b75c3/49

Answer 1

instead of where use having clause.
select `t`.*, GROUP_CONCAT(DISTINCT(s.name)) as skills,
GROUP_CONCAT(DISTINCT(s.id)) as skill_ids
FROM `candidates` `t`
LEFT JOIN `candidate-skills` `cs` ON `t`.`id` = `cs`.`can_id`
LEFT JOIN `skills` `s` ON `cs`.`skill_id` = `s`.`id`
GROUP BY `t`.`id`
having find_in_set ('8', skill_ids) and find_in_set ('10', skill_ids)
ORDER BY `t`.`id` desc
in Codeigniter
//take all skill ids in array
$ids=['8','10'];
$this->db->select("t.*");
$this->db->select("GROUP_CONCAT(DISTINCT(s.name)) as skills");
$this->db->select("GROUP_CONCAT(DISTINCT(s.id)) as skill_ids");
$this->db->from("candidates t");
$this->db->join("candidate-skills cs","t.id = cs.can_id");
$this->db->join("skills s","cs.skill_id = s.id");
$this->db->group_by("t.id");
foreach ($ids as $id) {
$this->db->having("find_in_set ('$id', skill_ids)");
}
$this->db->order_by("t.id","desc");
$query=$this->db->get();
$candidates=$query->result();

Answer 2

The most flexible way is using multiple JOINs; GROUP_CONCAT and comma delimited lists are considered an antipattern and it might not work if the concatenation isn't done exactly in the correct order (skill set 1,2,5 is considered not the same as 1,5,2).
SELECT c.* FROM candidates AS c
JOIN candidateskills AS cs ON (cs.cand_id = c.id)
JOIN skills AS sk1 ON (cs.skill_id = sk1.id)
JOIN skills AS sk2 ON (cs.skill_id = sk2.id)
...other sk(N)...
WHERE (sk1.skill = 'waterskiing')
AND (sk2.skill = 'snowboarding')
...
;
This allows easy tailoring of skills if, for example, each skill has a skill level and you need for snowboarding to be skilled at or above level 5. This kind of flexibility is a hell to do with GROUP_CONCAT.
But for simple matching, you can do it faster by selecting the skills you want and just counting them:
SELECT c.* FROM candidates AS c
JOIN candidateskills AS cs ON (cs.cand_id = c.id)
WHERE cs.skill_id IN (1, 7, 24, 19, 115)
GROUP BY c.id
HAVING COUNT(1) = 5;
(In more proper SQL you'd need to indicate explicitly all fields of c instead of "c.*", and repeat them in the GROUP BY clause. More clever RDBMS servers will not care as long as you group by c's primary key. MySQL currently does not care anyway, but in strict mode, it would).
For each skill you run a single, fast query on skills to retrieve its ID and assemble the query above.
Or you can do in a single, larger query as long as you have an exact match for the skill:
SELECT c.* FROM candidates AS c
JOIN candidateskills AS cs ON (cs.cand_id = c.id)
JOIN skills AS s ON (cs.skill_id = s.id)
WHERE s.skill IN ('javascript', 'html5', 'php')
GROUP BY c.id
HAVING COUNT(1) = 3;
Since you want this in PHP:
$skills = array('javascript', 'html5', 'php');
$skno = count($skills);
$set = implode(',', array_fill('?', $skno));
$params = $skills;
$params[] = $skno;
$query = "SELECT c.* FROM candidates AS c
JOIN candidateskills AS cs ON (cs.cand_id = c.id)
JOIN skills AS s ON (cs.skill_id = s.id)
WHERE s.skill IN ({$set})
GROUP BY c.id
HAVING COUNT(1) = ?";
$stmt = $db->prepare($query);
$stmt->execute($params);
while ($candidate = $stmt->fetch(PDO::FETCH_ASSOC)) {
...
}

Answer 3

Maybe this
select t.* ,
s.skills,s.skills_id
FROM `candidates` `t`
join
(
select t.id tid, group_concat(s.name) skills, group_concat(s.id order by s.id) skills_id
FROM `candidates` `t`
LEFT JOIN `candidate-skills` `cs` ON `t`.`id` = `cs`.`can_id`
LEFT JOIN `skills` `s` ON `cs`.`skill_id` = `s`.`id`
group by t.id
) s
on s.tid = t.id
where instr(skills_id,'8,10') > 0
+----+----------------+---------------------+-------------+------------+-------------------------------------------+-------------+
| id | name | created_on | modified_on | is_deleted | skills | skills_id |
+----+----------------+---------------------+-------------+------------+-------------------------------------------+-------------+
| 1 | Eugine | 2017-05-23 11:44:30 | NULL | N | zend framework 2,bootstrap,wordpress | 8,10,12 |
| 2 | Frinoy Francis | 2017-05-23 16:44:29 | NULL | N | html,html5,zend framework 2,php,bootstrap | 1,4,8,10,11 |
+----+----------------+---------------------+-------------+------------+-------------------------------------------+-------------+
2 rows in set (0.03 sec)
MariaDB [sandbox]> select t.* ,
-> s.skills,s.skills_id
-> FROM `candidates` `t`
-> join
-> (
-> select t.id tid, group_concat(s.name) skills, group_concat(s.id order by s.id) skills_id
-> FROM `candidates` `t`
-> LEFT JOIN `candidate-skills` `cs` ON `t`.`id` = `cs`.`can_id`
-> LEFT JOIN `skills` `s` ON `cs`.`skill_id` = `s`.`id`
-> group by t.id
-> ) s
-> on s.tid = t.id
-> where instr(skills_id,'') > 0
-> ;
+----+----------------+---------------------+-------------+------------+-------------------------------------------+-------------+
| id | name | created_on | modified_on | is_deleted | skills | skills_id |
+----+----------------+---------------------+-------------+------------+-------------------------------------------+-------------+
| 1 | Eugine | 2017-05-23 11:44:30 | NULL | N | zend framework 2,bootstrap,wordpress | 8,10,12 |
| 2 | Frinoy Francis | 2017-05-23 16:44:29 | NULL | N | html,html5,zend framework 2,php,bootstrap | 1,4,8,10,11 |
| 3 | Arun | 2017-05-28 12:56:24 | NULL | N | bootstrap | 8 |
+----+----------------+---------------------+-------------+------------+-------------------------------------------+-------------+
3 rows in set (0.03 sec)