题目详情

中级软件设计师,模拟考试,2021年软件设计师模拟题4

  • A.5
  • B.6
  • C.7
  • D.8

正确答案及解析

正确答案
C
解析

关系R(A,B,C,D,E)和S(B,C,F,G)做自然连接时,会以两个关系公共字段做等值连接,然后将操作结果集中重复列去除,所以运算后属性列有7个。

接下来分析关系表达式的SQL形式,题目中关系表达式先进行了R与S的自然连接。得到的结果集为:RS(R.A,R.B,R.C,R.D,R.E,S.F,S.G)。此后的选择操作“σ3<6”可表达为“σR.C<S.F”;最后进行投影操作“π1,3,6,7”即选出结果集的第1,3,6,7列,对应的列为:R.A,R.C,S.F,S.G(由于无重复字段,A,C,F,G及A,R.C,F,G或其它等价形式均可)。

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