问题
单项选择题
设有关系R(A,B,C)和S(A,D,E,F)，且R.A和S.A取自同一个域，与下面SQL语句： SELECT R.A,B,E,F FROM R,S WHERE R.A=S.A AND B=3等价的关系代数表达式是：
A． 只有Ⅰ
B． 只有Ⅱ
C． Ⅱ和Ⅲ
D． 都等价
答案
参考答案：D
解析：由投影和连接的关系运算规则可以得出答案。
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设有关系R(A,B,C)和S(A,D,E,F)，且R.A和S.A取自同一个域，与下面SQL语句： SELECT R.A,B,E,F FROM R,S WHERE R.A=S.A AND B=3等价的关系代数表达式是：
A． 只有Ⅰ
B． 只有Ⅱ
C． Ⅱ和Ⅲ
D． 都等价
参考答案：D
解析：由投影和连接的关系运算规则可以得出答案。
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