英语翻译
9.
Althoughprimarykeysarenotactuallyrequired,mostdatabasedesignersensurethateverytabletheycreatehasaprimarykeysothatfuturedatamanipulationispossibleandmanageable.Anycolumninatablecanbeestablishedastheprimarykey,aslongasitmeetsthefollowingconditions:Notworowscanhavethesameprimarykeyvalue.Everyrowmusthaveaprimarykeyvalue(primarykeycolumnsmaynotallowNULLvalues).Valuesinprimarykeycolumnscanneverbemodifiedorupdated.ourATMcardatacashmachine,youareusingdatabasesforPINnumberverificationandbalancechecking.