CREATE OR REPLACE TRIGGER MYUSER.TRIG_INS_MYTABLE
BEFORE INSERT ON MYUSER.TRIG_INS_MYTABLE FOR EACH ROW
WHEN (NEW.MYFIELDA IS NULL)
DECLARE
 v_fielda CHAR(4);
BEGIN
 SELECT MAX(fielda)
 INTO v_fielda
 FROM MYOTHERTABLE
 WHERE myothertable_id = :NEW.myothertable_id;
 :NEW.fielda := v_fielda;
END trig_ins_MYTABLE;
/