在关系模型中，关系的“元数”(arlty)是指 (21) 。有关系只和S，R ∩ S的运算等价于 (22) 。设关系R和S的属性个数分别为r和5，则(R×S)操作结果的属性个数为 (23) 。

A．多于7种
B．少于7种
C．7种
D．不正确

A．SELECT DISTINCTSname
B．SELECT DISTm CTSname
FROMS FROMS
WHERE EXISTS WHERE 0＜
(SELECT* (SELECTCOUNT(*)
FROM SPJ FROM SPJ
WHERE S．Sno=SPJ．SnO WHERE S．Sno；SPJ．Sno
AND SPJ．Pno；，P3， AND SPJ．Pno=’P3’)
C．SELECT Shame
D．SELECT Sname
FROMS FROMS，SPJ
WHERE EXISTS WHERES.Sno=SPJ.Sno
(SELECT* ANDSPJ．Pno=’P3’)
FROM SPJ GROUP BYSname
WHERE S. Sno=SPJ. Sno
AND SPJ. Pno=’P3’)