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.
A.SELECT * FROM SAILORS WHERE LASTNAME<>'NULL'
B.SELECT * FROM SAILORS WHERE LASTNAME<>NULL
C.SELECT * FROM SAILORS WHERE LASTNAME IS not NULL
D.SELECT * FROM SAILORS WHERE LASTNAME not LIKE 'NULL'
正确答案是C