MySQL 8.0

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CREATE TABLE skills (`id` INTEGER, `idUser` INTEGER, `idSkill` INTEGER);

INSERT INTO skills (`id`, `idUser`, `idSkill`) VALUES
  ('1', '4', '1'),
  ('2', '8', '4'),
  ('3', '8', '9'),
  ('4', '13', '9'),
  ('5', '18', '2'),
  ('6', '22', '1'),
  ('7', '27', '2'),
  ('8', '32', '4'),
  ('9', '11', '2'),
  ('10', '32', '9'),
  ('10', '32', '7');

Records: 11  Duplicates: 0  Warnings: 0


SELECT idUser
FROM skills 
WHERE idSkill IN (9, 4, 7) 
GROUP BY idUser 
HAVING SUM(idSkill IN (9, 4)) = 2 -- for the 2 mandatory skills


idUser


8

32


SELECT idUser
FROM skills 
WHERE idSkill IN (9, 4, 7) 
GROUP BY idUser 
HAVING SUM(idSkill IN (9, 4)) = 2 -- for the 2 mandatory skills
ORDER BY COUNT(*) DESC
LIMIT 1


idUser


32


SELECT idUser
FROM (
  SELECT idUser, RANK() OVER (ORDER BY COUNT(*) DESC) rnk
  FROM skills 
  WHERE idSkill IN (9, 4, 7) 
  GROUP BY idUser 
  HAVING SUM(idSkill IN (9, 4)) = 2 -- for the 2 mandatory skills
) t
WHERE rnk = 1


idUser


32