第27题: [单项选择]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=‘’ 参考答案:C 答案解析:IS NULL表示空值,空值不能表示为列名=NULL。
第33题: [单项选择]The student said there were a few points in the essay he ______ impossible to comprehend. A. ought to have B. was finding C. had found D. would find 参考答案:C 答案解析:[考点] 考查句子时态。 主句用的是一般过去时,所以从句中要用与过去对应的时态,由题意可知,“发现”的动作在“说”之前,故正确答案为C。