ТЕМА:

create table test (
    test_id number(9)
  , active_flag number(1)
  , constraint ch_test_active_flag
      check (active_flag is null or active_flag = 1)
)
/
insert into test(test_id, active_flag) values (1, null);
insert into test(test_id, active_flag) values (1, null);
insert into test(test_id, active_flag) values (1, 1);
insert into test(test_id, active_flag) values (2, null);
insert into test(test_id, active_flag) values (2, 1);
commit;