DECLARE
  x VARCHAR2(1);  
  y VARCHAR2(1);  
BEGIN
  IF (x is null and y is null) or x=y THEN
    dbms_output.put_line('YES');
  ELSE
    dbms_output.put_line('NO');
  END IF;
END;

------解决方案--------------------
SQL code

--不等的情况,还真没太在意过，呵呵
DECLARE
  x VARCHAR2(1);  
  y VARCHAR2(1);  
BEGIN
  IF (x is null and y is not null) or (x is not null and y is null) or x<>y THEN
    dbms_output.put_line('not equal');
  ELSE
    dbms_output.put_line('equal');
  END IF;
END;
/

------解决方案--------------------
不等于的情况，用这个
SQL code


DECLARE
  x VARCHAR2(1); 
  y VARCHAR2(1); 
BEGIN
  IF (x||'1' <> y||'1')THEN  --NULL做隐式转化
  dbms_output.put_line('NO');
  ELSE
  dbms_output.put_line('YES');
  END IF;
END;