题目内容

对于教学数据库的三个基本表：S(Sno，Sname，Ssex)，SC(Sno，Cno，Grade)，C(Cno，Cname， Teacher)，其中S为学生表，SC为学生选课表，C为课程表，则对于语句：检索至少选修LIU老师所授课课程中一门课程的女生姓名，SQL语句错误的是______。

A. SELECT Sname FROM S，SC，C WHERE Sno=SSno AND SCno=Cno AND Teacher=‘LIU’AND Ssex=‘F’
B. SELECT Sname FROM S WHERE Ssex=‘F’AND Sno IN (SELECT Sno FROM SC WHERE Cno IN(SELECT Cno FROM C WHERE Teacher=‘LIU’))
C. SELECT Sname FROM S WHERE Ssex=‘F’AND EXISTS (SELECT* FROM SC WHERE SSno=Sno AND EXISTS (SELECT* FROM C WHERE Teacher=‘LIU’))
D. SELECT Sname FROM S WHERE Ssex=‘F’AND EXISTS(SELECT* FROM SC WHERE SSno=Sno AND EXISTS (SELECT* FROM C WHERE Cno=SCno AND Teacher=‘LIU’))

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关系SC(Sno，Cno，Grade)表示学号，课程号，成绩。现要查询平均分在90分以上的学生的学号和平均成绩，正确的语句是______。

A. SELECT Sno，AVG(Grad FROM SC WHERE AVG(Grad＞=90 GROUP BY Sno
B. SELECT Sno，AVG(Grad FROM SC GROUP BY Sno HAVING AVG(Grad＞=90
C. SELECT Sno，AVG(Grad FROM SC WHERE AVG(Grad＞=90 ORDER BY Sno
D. SELECT Sno，AVG(Grad FROM SC GROUP BY Cno HAVING AVG(Grad＞=90

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A secretary should take notes（）.

A. after the meeting
B. during a meeting,
C. when they are discussing before the meeting
D. in all the cases mentioned

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（）the book again, she discovered that she had missed a lot in her first reading.

A. While reading
B. When read
C. As reading
D. In reading

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I had worked here （）you came here. But l shall leave for England（）.

A. before long; before long
B. before long; long before
C. long before; before long
D. long before; long before

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