37.第(37.-(38.题基于学生-课程数据库中的三个基本表:
学生信息表:s(sno, sname, sex, age, dept. 主码为sno
课程信息表:c(cno, cname, teacher. 主码为cno
学生选课信息表:sc(sno, cno, grade. 主码为(sno, cno.
"从学生选课信息表中找出无成绩的元组"的SQL语句是(本题分值:1分)
(A)SELECT * FROM sc WHERE grade=NULL
(B)SELECT * FROM sc WHERE grade IS ' '
(C)SELECT * FROM sc WHERE grade IS NULL
(D)SELECT * FROM sc WHERE grade =' '