Oracle入門–select構文で不等号(<>)とNullの検索結果

SQLコマンド
select * from scott.user t;  —-All data

select * from scott.user t where t.ctm != 800;–Not equal 800(Not contain null)

select * from scott.user t where t.ctm != 800 or t.ctm is null;–Not equal 800(Does contain null)