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【单选题】
下列关于主键、外键和候选键的说法中,正确的是A.
外键所引用的属性必须先定义为主键或候选键
B.
外键与其引用的主键应分别属于不同的关系
C.
一个关系中只能定义一个候选键
D.
候选键就是主键
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