De acordo com o diagrama do esquema de banco de dados relaci...

SELECT c.nom_corretor FROM CORRETOR AS c
WHERE c.id_corretor EXISTS
(SELECT id_corretor FROM TELEFONE)
AND c.id_corretor NOT EXISTS
(SELECT id_corretor FROM ATENDE)

SELECT c.nom_corretor FROM CORRETOR AS c
WHERE c.id_corretor IN
(SELECT id_ telefone FROM TELEFONE)
AND c.id_corretor NOT IN
(SELECT id_ regiao FROM ATENDE)

SELECT c.nom_corretor FROM CORRETOR AS c
WHERE c.id_corretor IN
(SELECT id_ corretor FROM TELEFONE)
AND c.id_corretor NOT IN
(SELECT id_ corretor FROM ATENDE)

SELECT c.nom_corretor FROM CORRETOR AS c
INNER JOIN TELEFONE AS t
      ON c.id_corretor = t.id_corretor
INNER JOIN ATENDE AS A
      ON c.id_corretor = a.id_corretor
WHERE a.id_corretor IS NULL
AND t.id_corretor IS NOT NULL