员工表staff表结构如下 staffNo varchar2(10), Email varchar2(50) 哪一个SQL语句能查询出没有E_mail地址的员工号（C ） A. select staffno from Staff where Email = NULL B. select staffno from Staff where Email <> NULL C. select staffno from Staff where Email is null D. select staffno from Staff where Email is not null

员工表staff表结构如下
staffNo varchar2(10),
Email varchar2(50)
哪一个SQL语句能查询出没有E_mail地址的员工号（C ）
A. select staffno from Staff where Email = NULL
B. select staffno from Staff where Email <> NULL
C. select staffno from Staff where Email is null
D. select staffno from Staff where Email is not null

出自：国家开放大学MySQL数据库应用

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