SELECT FNAME, LNAME
FROM EMPLOYEE
WHERE SUPERSSN IS NULL;

SELECT E.FNAME, E.LNAME
FROM EMPLOYEE AS E
WHERE E.SSN IN 
  (SELECT D.ESSN
   FROM DEPENDENT AS D
   WHERE E.FNAME=D.DEPENDENT_NAME);

SELECT E.FNAME, E.LNAME
FROM EMPLOYEE E
WHERE NOT EXISTS (SELECT *
                  FROM DEPENDENT D
                  WHERE E.SSN=D.ESSN);

SELECT E.FNAME, E.LNAME, D.DNAME
FROM (DEPARTMENT AS D LEFT OUTER JOIN EMPLOYEE AS E ON D.DNUMBER=E.DNO);

SELECT PNUMBER, PNAME, COUNT(*)
FROM PROJECT, WORKS_ON
WHERE PNUMBER=PNO
GROUP BY PNUMBER, PNAME
HAVING COUNT(*) > 2;

CREATE ASSERTION SALARY_CONSTRAINT
CHECK (NOT EXISTS (SELECT *
                   FROM EMPLOYEE E, EMPLOYEE M, DEPARTMENT D
                   WHERE E.SALARY > M.SALARY AND
                         E.DNO=D.DNUMBER AND 
                         D.MGRSSN=M.SSN));

CREATE TRIGGER INFORM_SUPERVISOR
BEFORE INSERT OR UPDATE OF SALARY, SUPERVISOR_SSN ON EMPLOYEE
FOR EACH ROW
WHEN (NEW.SALARY > (SELECT SALARY FROM EMPLOYEE WHERE SSN=NEW.SUPERVISOR_SSN))
INFORM_SUPERVISOR (NEW.SUPERVISOR_SSN, NEW.SSN);