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假设供应商S和供应情况SPJ的关系模式分别为:S(Sno,Sname,Status,City)和SPJ(Sno,Pno,Jno,Qty)。SQL语句(22)不能正确地查询出“零件号Pno等于‘P3’的供应商名Sname”,而(23)能正确查询的关系代数表达式。

  • A.SELECT  DISTINCT Sname       FROMS        WHERE EXISTS(SELECT*        FROM SPJ        WHERE S. Sno=SPJ. SnO        AND SPJ.Pno='P3')
  • B.SELECT    DISTmCT Sname       FROMS       WHERE 0<(SEIECT COUNT(*)FROM SPJ       WHERES.Sno=SPJ.Sno   AND SPJ.Pno='P3')
  • C.SELECT Sname       FROMS       WHERE EXISTS(SELECT*       FROM SPJ       WHERES.Sno=SPJ.Sno       ANDSPJ.Pno='P3')
  • D.SELECT Sname       FROMS,SPJ       WHERES.Sno=SPJ.Sno       ANDSPJ.Pno='P3')GROUP BYSname
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