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单项选择题

A.SELECT sno,cno FROM sc WHERE grade=NULL B.SELECT sno,……

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:()

A.SELECT sno,cno FROM sc WHERE grade=NULL
B.SELECT sno,cno FROM sc WHERE grade IS‘’
C.SELECT sno,cno FROM sc WHERE grade IS NULL
D.SELECT sno,cno FROM sc WHERE grade=‘’

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