已知学生选课信息表
sc(sno,cno,grade)查询“至少选修了一门课程，但没有学习成绩的学生学号和课程号”的SQL语句是( )
SELECT sno,cno FROM sc WHERE grade = NULL
SELECT sno,cno FROM sc WHERE grade IS ''
SELECT sno,cno FROM sc WHERE grade IS NULL
SELECT sno,cno FROM sc WHERE grade = ''
判断NULL使用"is NULL"
