CREATE TABLE MYTABLE
  ( FLDA  CHAR(2)  NOT NULL,
    FLDB  CHAR(2)          ,
    FLDC  CHAR(2)            ) ;

INSERT INTO MYTABLE VALUES ('A1', 'B1', 'C1');
INSERT INTO MYTABLE VALUES ('A2', 'B2', NULL);
INSERT INTO MYTABLE VALUES ('A3', NULL, 'C3');
INSERT INTO MYTABLE VALUES ('A4', NULL, NULL);
INSERT INTO MYTABLE VALUES ('A5', 'B5', 'C5');
                                              
SELECT * FROM MYTABLE;