单项选择题
第(23)第(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Πsname(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"