单项选择题
23～24 题基于以下说明。设有三个关系：
学生关系S(SNO,SNAME,AGE,SEX) (分别代表学生的学号、姓名、年龄、性别)
学习关系SC(SNO,CNO,GRADE) (分别代表学生的学号、课程编号、成绩)
课程关系C(CNO,CNAME,TEACHER) (分别代表课程的编号、课程名、任课教师)
下列不能实现与关系代数表达式Ⅱsname(σcname=‘DB’(S
SC
C))相同查询的SQL命令是
A．SELECT SNAME FROM S,SC,C WHERE S.SNO=SC.SNO AND SC.CNO=C.CNO AND CNAME=′DB′
B．SELECT SNAME FROM S WHERE SNO IN(SELECT SNO FROM SC,C WHERE SC.CNO=C.CNO AND CNAME=′DB′)
C．SELECT SNAME FROM S WHERE SNO IN(SELECT SNO FROM SC WHERE CNO IN(SELECT CNO FROM C WHERE CNAME=′DB′) )
D．SELECT SNAME FROM S,C WHERE CNAME=′DB′
B．SELECT SNAME FROM S WHERE SNO IN(SELECT SNO FROM SC,C WHERE SC.CNO=C.CNO AND CNAME=′DB′)
C．SELECT SNAME FROM S WHERE SNO IN(SELECT SNO FROM SC WHERE CNO IN(SELECT CNO FROM C WHERE CNAME=′DB′) )
D．SELECT SNAME FROM S,C WHERE CNAME=′DB′