关系模式Sailors(sid,lastname.firtname,age,ratring)其中lastname允许为空. 下面哪个语句能够查找出lastname属性值不为空的水手信息.

ASELECT * FROM SAILORS WHERE LASTNAME‘NULL‘

BSELECT * FROM SAILORS WHERE LASTNAMENULL

CSELECT * FROM SAILORS WHERE LASTNAME IS not NULL

DSELECT * FROM SAILORS WHERE LASTNAME not LIKE ‘NULL‘