设有关系R=(A,B,C),与SQL语句SELECT DISTINCT A FROM R WHERE B =17等价的关系代数表达式是( )。A.ЛA(R)B.σB=17(R)C.ЛA(σB=17(R))D.σB=17(ЛA(R))

设有关系R=(A,B,C),与SQL语句SELECT DISTINCT A FROM R WHERE B =17等价的关系代数表达式是( )。

A.ЛA(R)

B.σB=17(R)

C.ЛA(σB=17(R))

D.σB=17(ЛA(R))


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