A.select to_date(sysdate,'YYYY-MM-DD HH24:MI:SS') from dual;
B.select to_char(sysdate,'YYYY-MM-DD HH24:MI:SS') from dual;
C.select date(sysdate,'YYYY-MM-DD HH24:MI:SS') from dual;
D.select chr(sysdate,'YYYY-MM-DD HH24:MI:SS') from dual;
A.A UNION B
B.A UNION ALL B
C.A INTERSECT B
D.A MIUS B
A.A MIUS B
B.B MINUS A
C.A INTERSECT B
D.B INTERSECT A