‌下列SQL语句中，________可以在“学生-课程”数据库中实现“查询所有有成绩的学生学号和课程号”。​ A: SELECT Sno，CnoFROM SCWHERE Grade NOT NULL B: SELECT Sno，CnoFROM SCWHERE Grade <> NULL C: SELECT Sno，CnoFROM SCWHERE Grade IS NOT NULL D: SELECT Sno，CnoFROM SCWHERE Grade >0

查询计算机系任意学生选修过的课程号，不正确的查询语句是： A: SELECT DISTINCT CnoFROM SCWHERE NOT EXISTS (SELECT SnoFROM StudentWHERE Sdept = 'CS'AND SSno = Student.Sno); B: SELECT DISTINCT CnoFROM SCWHERE EXISTS (SELECT SnoFROM StudentWHERE Sdept = 'CS'AND SSno = Student.Sno); C: SELECT DISTINCT CnoFROM SCWHERE Sno IN (SELECT SnoFROM StudentWHERE Sdept = 'CS'); D: SELECT DISTINCT CnoFROM SCLEFT JOIN StudentON SSno = Student.SnoWHERE Sdept = 'CS';

查询计算机系任意学生选修过的课程号，不正确的查询语句是： A: SELECT DISTINCT CnoFROM SCWHERE NOT EXISTS (SELECT SnoFROM StudentWHERE Sdept = 'CS'AND SC.Sno = Student.Sno); B: SELECT DISTINCT CnoFROM SCWHERE EXISTS (SELECT SnoFROM StudentWHERE Sdept = 'CS'AND SC.Sno = Student.Sno); C: SELECT DISTINCT CnoFROM SCWHERE Sno IN (SELECT SnoFROM StudentWHERE Sdept = 'CS'); D: SELECT DISTINCT CnoFROM SCLEFT JOIN StudentON SC.Sno = Student.SnoWHERE Sdept = 'CS';

‍下列SQL语句中，_____________可以在“学生-课程”数据库正确实现“找出每个学生超过他选修课程平均成绩的课程号”。‍ A: SELECT Sno，CnoFROM SC xWHERE Grade >=(SELECT AVG(Grade) FROM SC y WHERE y.Sno=x.Sno) B: SELECT Sno，CnoFROM SC xWHERE (SELECT AVG(Grade) FROM SC y WHERE y.Sno=x.Sno)<=Grade C: SELECT Sno，CnoFROM SC WHERE Grade >=(SELECT AVG(Grade) FROM SC ) D: SELECT x.Sno，x.CnoFROM SC x, SC yWHERE y.Sno=x.Sno AND x.Grade >=(SELECT AVG(Grade) FROM y)

查询计算机系任意学生选修过的课程号，可以使用如下语句：SELECT DISTINCT CnoFROM SCWHERE Sno IN (SELECT SnoFROM StudentWHERE Sdept = 'CS');

查询学号为201215121的同学选修过的所有课程的名字，不正确的查询语句是： A: SELECT CnameFROM CourseLEFT JOIN SCON Course.Cno = SCnoWHERE Sno = '201215121'; B: SELECT CnameFROM CourseWHERE Cno IN (SELECT CnoFROM SCWHERE Sno = '201215121'); C: SELECT CnameFROM CourseWHERE EXISTS (SELECT *FROM SCWHERE SCno = Course.CnoAND Sno = '201215121'); D: SELECT CnameFROM CourseWHERE NOT EXISTS (SELECT *FROM SCWHERE SCno = Course.CnoAND Sno = '201215121');