Esta é uma Divisão Relacional sem Restante (RDNR) problema. Veja este artigo por Dwain Camps que fornece muitas soluções para este tipo de problema.
Primeira solução
SQL Fiddle
SELECT empId
FROM (
SELECT
empID, cc = COUNT(DISTINCT department)
FROM employe
WHERE department IN('Y', 'Z')
GROUP BY empID
)t
WHERE
t.cc = 2
AND t.cc = (
SELECT COUNT(*)
FROM employe
WHERE empID = t.empID
)
Segunda Solução
SQL Fiddle
SELECT e.empId
FROM employe e
WHERE e.department IN('Y', 'Z')
GROUP BY e.empID
HAVING
COUNT(e.department) = 2
AND COUNT(e.department) = (SELECT COUNT(*) FROM employe WHERE empID = e.empId)
Sem usar
GROUP BY e HAVING :SELECT DISTINCT e.empID
FROM employe e
WHERE
EXISTS(
SELECT 1 FROM employe WHERE department = 'Z' AND empID = e.empID
)
AND EXISTS(
SELECT 1 FROM employe WHERE department = 'Y' AND empID = e.empID
)
AND NOT EXISTS(
SELECT 1 FROM employe WHERE department NOT IN('Y', 'Z') AND empID = e.empID
)
