DROP TABLE IF EXISTS appointments;
CREATE TABLE appointments (
  id        integer,
  title     varchar(255),
  created_at date
);

DROP TABLE IF EXISTS appointment_dates;
CREATE TABLE appointment_dates (
  id        integer,
  appointment_id integer,
  confirmed boolean
);

INSERT INTO appointments VALUES (1, 'one', '2018-05-21');
INSERT INTO appointment_dates VALUES (1, 1, false);
INSERT INTO appointment_dates VALUES (2, 1, true);

INSERT INTO appointments VALUES (2, 'two', '2018-05-21');
INSERT INTO appointment_dates VALUES (3, 2, false);
INSERT INTO appointment_dates VALUES (4, 2, false);

INSERT INTO appointments VALUES (3, 'three', '2018-05-22');
INSERT INTO appointment_dates VALUES (4, 3, false);
INSERT INTO appointment_dates VALUES (5, 3, false);

INSERT INTO appointments VALUES (4, 'four', '2018-05-21');

SELECT *
FROM appointments a
LEFT JOIN appointment_dates ad
  ON a.id = ad.appointment_id
  AND ad.confirmed = true
WHERE a.created_at = '2018-05-21'
  AND ad.id IS NULL;