gistfile1.txt

queries:
SELECT t.name, COUNT(*) AS nb_player
  FROM player as p
  JOIN team AS t ON p.team_id=t.id
  GROUP by t.id
  ORDER BY nb_player DESC;

SELECT t.name
  FROM player as p
  JOIN team AS t ON p.team_id=t.id
  GROUP by t.id
  HAVING COUNT(*) > 14
  ORDER BY t.name;

SELECT CONCAT (w.firstname, ' ', w.lastname) AS fullname, WEEKDAY(p.enrollment_date)
  FROM player as p
  JOIN wizard AS w ON p.wizard_id=w.id
  JOIN team AS t ON p.team_id=t.id
  WHERE t.name = 'Gryffindor' AND WEEKDAY(p.enrollment_date) = 0
  ORDER BY p.enrollment_date;

result:

mysql> source quest-520.sql;
+------------+-----------+
| name       | nb_player |
+------------+-----------+
| Gryffindor |        36 |
| Slytherin  |        21 |
| Ravenclaw  |        15 |
| Hufflepuff |        12 |
+------------+-----------+
4 rows in set (0,00 sec)

+------------+
| name       |
+------------+
| Gryffindor |
| Ravenclaw  |
| Slytherin  |
+------------+
3 rows in set (0,00 sec)

+--------------+----------------------------+
| fullname     | WEEKDAY(p.enrollment_date) |
+--------------+----------------------------+
| fred weasley |                          0 |
+--------------+----------------------------+
1 row in set (0,00 sec)