两条直线相交成直角，就是说这两条直线互相______，其中一条直线叫做另一条直线的______．这两条直线的交点叫做______．

假设供应商S和供应情况SPJ的关系模式分别为S(Sno，Snaale，Status，City)和SPJ (Sno，Pno，Jno，Qty)。SQL。语句 (19) 不能正确地查询㈩“零件号Pno等于‘P3’的供应商名Snam”，而 (20) 能正确查询的关系代数表达式。

A．SELECT DISTINCTSname

B．SELECT DISTm CTSname
FROMSFROMS
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’)