طريقة الاستعلام عن التواريخ المتتالية فى جدول

CREATE TABLE vacations
(employee_id NUMBER, vac_date DATE);

INSERT INTO vacations VALUES (15,to_date('2017/01/01','yyyy/mm/dd'))
​;INSERT INTO vacations VALUES (15,to_date('2017/01/02','yyyy/mm/dd'));
INSERT INTO vacations VALUES (15,to_date('2017/01/06','yyyy/mm/dd'));
INSERT INTO vacations VALUES (17,to_date('2017/01/03','yyyy/mm/dd'));
INSERT INTO vacations VALUES (17,to_date('2017/01/07','yyyy/mm/dd'));
INSERT INTO vacations VALUES (17,to_date('2017/01/20','yyyy/mm/dd'));
INSERT INTO vacations VALUES (15,to_date('2017/01/15','yyyy/mm/dd'));
INSERT INTO vacations VALUES (17,to_date('2017/01/19','yyyy/mm/dd'));
INSERT INTO vacations VALUES (17,to_date('2017/02/17','yyyy/mm/dd'));
INSERT INTO vacations VALUES (15,to_date('2017/01/21','yyyy/mm/dd'));
INSERT INTO vacations VALUES (15,to_date('2017/01/22','yyyy/mm/dd'));
INSERT INTO vacations VALUES (17,to_date('2017/02/05','yyyy/mm/dd'));
INSERT INTO vacations VALUES (17,to_date('2017/01/23','yyyy/mm/dd'));
INSERT INTO vacations VALUES (15,to_date('2017/02/25','yyyy/mm/dd'));
INSERT INTO vacations VALUES (15,to_date('2017/02/28','yyyy/mm/dd'));
INSERT INTO vacations VALUES (17,to_date('2017/02/26','yyyy/mm/dd'));
COMMIT;