CREATE TABLE student
(
  sname CHARACTER VARYING,
  pstatus INTEGER,
  sgrade CHARACTER VARYING
);

INSERT INTO student(sname, pstatus, sgrade) VALUES ('MR.A', 1, 'A');
INSERT INTO student(sname, pstatus, sgrade) VALUES ('MR.B', 1, 'A');
INSERT INTO student(sname, pstatus, sgrade) VALUES ('MR.C', NULL, 'B');
INSERT INTO student(sname, pstatus, sgrade) VALUES ('MR.D', 1, NULL);
INSERT INTO student(sname, pstatus, sgrade) VALUES ('MR.E', NULL, NULL);
INSERT INTO student(sname, pstatus, sgrade) VALUES ('MR.F', 1, 'A');
INSERT INTO student(sname, pstatus, sgrade) VALUES ('MR.G', NULL, 'A');
INSERT INTO student(sname, pstatus, sgrade) VALUES ('MR.H', 1, 'C');
INSERT INTO student(sname, pstatus, sgrade) VALUES ('MR.I', 2, NULL);
INSERT INTO student(sname, pstatus, sgrade) VALUES ('MR.J', 2, 'A');
INSERT INTO student(sname, pstatus, sgrade) VALUES ('MR.K', 3, 'A');
INSERT INTO student(sname, pstatus, sgrade) VALUES ('MR.L', 2, 'C');