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＝NULLB、select staffno from staff where E_mail NULLC、select staffno from staff where E_mail is NULLD、select staffno from staff where E_mail is not NULL
题目
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