下列不能实现与关系代数表达式Πsname(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"