DECLARE
   x   BOOLEAN;
   y   BOOLEAN;

   PROCEDURE show_truth (str IN VARCHAR2, val IN BOOLEAN)
   IS
   BEGIN
      DBMS_OUTPUT.put_line (
            str
         || ' - '
         || CASE val
               WHEN TRUE THEN 'TRUE'
               WHEN FALSE THEN 'FALSE'
               ELSE 'NULL'
            END);
   END;

   PROCEDURE show_line (x_in IN BOOLEAN, y_in IN BOOLEAN)
   IS
   BEGIN
      show_truth ('x and y', x_in AND y_in);
      show_truth ('x or y', x_in OR y_in);
      show_truth ('not x', NOT x_in);
      DBMS_OUTPUT.put_line ('---------------');
   END;
BEGIN
   show_line (TRUE, TRUE);
   show_line (TRUE, FALSE);
   show_line (TRUE, NULL);
   --
   show_line (FALSE, TRUE);
   show_line (FALSE, FALSE);
   show_line (FALSE, NULL);
   --
   show_line (NULL, TRUE);
   show_line (NULL, FALSE);
   show_line (NULL, NULL);
END;