A.空值等同于空字符串 B.空值表示字段或變量還沒有確定值 C.VFP不支持空值 D.空值等同于數(shù)值O
A.SELECT*FROM訂單WHERE SUBSTR(訂購單號,4)=,,1” B.SELECT*FROM訂單WHERE SUBSTR(訂購單號,4,1卜,,1” C.SELECT*FROM訂單WHERE”1”$訂購單號 D.SELECT*FROM訂單WHERE RIGHT(訂購單號,1)="1”
A.ALTER TABLE stock股票名稱WITH c(10) B.ALTER TABLE stock股票名稱c(10) C.ALTER TABLE stock ALTER股票名稱c(10) D.ALTER stock ALTER股票名稱c(10)