下列()是SQL语言查询出员工E_mail地址的语句。 A.select staff
问题:
下列()是SQL语言查询出员工E_mail地址的语句。
A.select staffno from staff where E_mail=NULL
B.select staffno from staff where E_mail NULL
C.select staffno from staff where E_mail is NULL
D.select staffno from staff where E_mail is not NULL