?????????????????a not in (b??c??null)??????????
???????????????????д?????????????????????????????????????????е???????塣
????????????????NULL???У???????????μ?????select * from tb where col=null
?????????????????????????????????????col is null ?????????漰??????????
??????????
??????SQL???????????????????TRUE??FALSE??UNKNOWN???????????????????
????????????SQL?????е?????????????????????????TRUE??FALSE???????
????SQL?е?UNKNOWN????????????????NULL???????????У????磬???????????????????UNKNOWN??
????NULL<42;
????NULL=NULL;
????X+NULL>Y;
????UNKNOWN????????????????????????true????false?????????????UNKNOWN????漴????????棬UNKNOWN????true??false?е????????UNKNOWN????????UNKNOWN
??????onwhere??having???????????????UNKNOWN????false????check?б?????true??????????????null???????UNKNOWN??????????а??????CHECK????????salary?е?????????0?????????salary?NULL????????????????NULL>0??????UNKNOWN????check????б??????TRUE?????
???????????б??????NULL??????UNKNOWN??????????false????????????????NULL???????????NULL??
??????UNIQUE???????????????????????????NULL????????
???????????????ж?????UNIQUE??????????????в????????NULL?????С?
????GROUP BY ????????NULL??????顣
????ORDER BY ????????NULL??????????
????????????????????null??????????? is null ????is not null ????????????????????????
???????
???????????????????

????????