有如下的SQL语句： Ⅰ.SELECT sname FROM s, sc WHERE grade<60 Ⅱ.SELECT sname FROM s WHERE sno IN(SELECT sno FROM sc WHERE grade<60) Ⅲ.SELECT sname FROM s, sc WHERE s.sno=sc.sno AND grade<60 若要查找分数(grade)不及格的学生姓名(sname)，则以上正确的有哪些？（ ） A: Ⅰ和Ⅱ B: Ⅰ和Ⅲ C: Ⅱ和Ⅲ D: Ⅰ、Ⅱ和Ⅲ

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=

在学生选课数据库中，查询平均成绩大于等于60分的同学，SQL语句为（ ） A: SELECT Sno, AVG(Grade) FROM SC GROUP BY Sno HAVING AVG(Grade)>=60; B: SELECT Sno FROM SC WHERE Grade>=60 GROUP BY Sno; C: SELECT Sno, Grade FROM SC GROUP BY Sno HAVING AVG(Grade)>=60; D: SELECT Sno FROM SC GROUP BY Sno HAVING Grade >=60;

检索选修了课程”C1”的学生中成绩最高的学生的学号，正确的SELECT语句为（ ） A: SELECT Sno FROM SC WHERE Cno=’C1’ AND GRADE >=ALL (SELECT Grade FROM SC WHERE Cno=’C1’); B: SELECT Sno FROM SC WHERE Cno=’C1’ AND GRADE IN (SELECT Grade FROM SC WHERE Cno=’C1’); C: SELECT Sno FROM SC WHERE Cno=’C1’ AND GRADE > ANY (SELECT Grade FROM SC WHERE Cno=’C1’); D: SELECT Sno FROM SC WHERE Cno=’C1’ AND GRADE >= (SELECT Grade FROM SC WHERE Cno=’C1’);

查询曾经考过100分的学生的学号，可以使用的SQL语句是：（ ） A: SELECT ALL Sno FROM SC WHERE Grade = 100; B: SELECT DISTINCT Sno FROM SC WHERE Grade = 100; C: SELECT ALL Sno FROM Student WHERE SGrade = 100; D: SELECT DISTINCT Sno FROM Student WHERE SGrade = 100;