-- UNION
SELECT s.ime,s.datum_rojstva
FROM student s
WHERE s.datum_rojstva='2017-11-16 18:27:30'
UNION
SELECT d.ime,d.Datum_r
FROM delavec d
WHERE d.Datum_r='2017-11-16 18:27:30';

-- UNION ALL

SELECT s.ime,s.datum_rojstva
FROM student s
WHERE s.datum_rojstva='2017-11-16 18:27:30'
UNION ALL
SELECT d.ime,d.Datum_r
FROM delavec d
WHERE d.Datum_r='2017-11-16 18:27:30';