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???????????????????Varchar(5)????????????????5?????
?????5.Varchar(5)??????????????20???
???????????????Varchar???????4??????????????????????????????д洢?????????С??5?????????char??????varchar?????????
????????Null?????
????????Null???????????????顣??Щ????鯔?????Null????????ó?Not Null+Default??????????????SQL Server?????????????????(TRUE??FALSE??UNKNOWN)????????????(True??False)?????????????????????IsNull???????滻Null???
????????????????Щ??????????????????Null???????????????????Not Null+Default?????????????????
???????Null??????????????????????????
????????????????(Sparse)
???????????? Null ??????????洢?????????С? ????м????? Null ?????????????????????? Null ??????????? ???????????? 20% ?? 40% ?????????????????????С?
???????????SSMS?е????????6?????
?????6.?????
????????????????????????????ο?MSDN??
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