MySQL 8.0

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CREATE TABLE tablename (`day` INTEGER, `person` VARCHAR(4), `amount` INTEGER);

INSERT INTO tablename (`day`, `person`, `amount`) VALUES
  ('1', 'John', '4'),
  ('1', 'Sam', '6'),
  ('2', 'John', '3'),
  ('3', 'John', '3'),
  ('3', 'Sam', '5'),
  ('4', 'John', '3');

Records: 6  Duplicates: 0  Warnings: 0


SELECT person,
       100 * AVG(amount > 3) percent
FROM tablename
GROUP BY person;


person
percent


John
25.0000

Sam
100.0000


SELECT person,
       100 * AVG(amount > 5) percent
FROM tablename
GROUP BY person;


person
percent


John
0.0000

Sam
50.0000