A . ΠA(R)
B . σB=17(R)
C . ΠA(σB=17(R))
D . σB=17(ΠA(R))
[单选题]设有关系R=(A,B,C)。与SQL语句SELECT DISTINCT A FROMR WHERE B=17等价的关系代数表达式是( )。A.πA^(R)B.πB=17(R)C.πA(σB=17^(R))D.σB=17(πA^(R))
[单选题]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.πA,B (R) B.σB=17 (R)C. πA(σB=17(R))D. σB=17(πA(R))
[单选题]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.πA(R)B.σB=17(R)C.πA(σB=17(R))D.σB=17(πA(R))
[单选题]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))
[单选题]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.ЛB=17(R)C.ЛA(σB=17(R))D.σB=17(ЛA(R))
[单选题]设有关系R=(A, B, C) 与SQL语句select distinct A, C from R where B=5等价的关系代数表达式是( )。A.都等价B.仅ⅠC.仅ⅡD.都不等价
[单选题]设有关系R(A, B, C) ,与SQL语句select distinct A, C from R where B=5等价的关系代数表达式是( )。Ⅰ. πA, C(σB=5(R)) Ⅱ. σB=5(πA, C(R))A.) 都等价B.) 仅ⅠC.) 仅ⅡD.) 都不等价A.B.C.D.
[单选题]设关系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.πA(σB=17(R))B.σB=17(πA(R))C.σB=17(πA,C(R))D.πA,C(σB=17(R))
[单选题]设有关系R:(A,B,C) 与SQL语句select distinct A,C from R where B=5等价的关系代数表达式是 Ⅰ.πA.C(σB=5(R)) Ⅱ.σB=5(πA.C(R))A.都等价B.仅ⅠC.仅ⅡD.都不等价
[单选题]设有关系R=(A,B,C) ,与SQL语句select distinct A,C from R where B=5等价的关系代数表达式是?A.都等价B.仅ⅠC.仅ⅡD.都不等价