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');

同时选修了‘数据结构’和‘操作系统’2门课程的同学学号、姓名。select student.sno,sname from student,sc,coursewhere student.sno=sc.sno and course.cno=sc.cnoand _________________________________________________ A: cname='数据结构' and student.sno in(select sno from sc,course where sc.cno=course.cno and cname='操作系统') B: cname='操作系统' and student.sno in(select sno from sc,course where sc.cno=course.cno and cname='数据结构') C: cname='数据结构' and cname='操作系统' D: student.sno in(select sno from sc,course where sc.cno=course.cno and cname='操作系统'and cname='数据结构')