单选

以下对于数据库的几种关键字的描述中,错误的是(20)。

  • A.主关键字一定是候选关键字
  • B.候选关键字一定是超关键字
  • C.超关键字—定是合成关键字
  • D.外部关键字一定是某个关系的候选关键字
参考答案
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