下列不能实现与关系代数表达式Ⅱsname(σcname=‘DB’(SSCC))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′