a)
SELECT f.Name, ifnull(p.Phone, '-')
FROM Firms f
         LEFT JOIN Phones p ON f.ID = p.FirmID;

b)
SELECT f.Name
FROM Firms f
         LEFT JOIN Phones p ON f.ID = p.FirmID
WHERE p.Phone IS NULL;

c)
SELECT f.Name
FROM Firms f
         LEFT JOIN Phones p ON f.ID = p.FirmID
GROUP BY f.Name
HAVING count(p.phone_Id) >= 2;

d)
SELECT f.Name
FROM Firms f
         LEFT JOIN Phones p ON f.ID = p.FirmID
GROUP BY f.Name
HAVING count(p.phone_Id) < 2;

e)
SELECT f.Name, count(p.phone_Id) AS phones_count
FROM Firms f
         LEFT JOIN Phones p ON f.ID = p.FirmID
GROUP BY f.Name
ORDER BY phones_count DESC
LIMIT 1;