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【单选题】
在满足实体完整性约束的条件下()A.
一个关系中只能有一个候选关键字
B.
一个关系中必须有多个候选关键字
C.
一个关系中可以没有候选关键字
D.
一个关系中应该有一个或多个候选关键字
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