把表Book中PubComp的类型varchar()改为varchar()AALTERTABLEBookALTERCOLUMNPubCompvarchar()NOTNULLBALTERTABLEBookALTERCOLUMNPubCompvarchar()tovarchar()CuseBookALTERCOLUMNPubCompvarchar()D其余选项全错