题目内容

有关主键和外键的描述,正确的是( )。

A. 一个表中至少有一个外键约束
B. 一个表中至少有一个主键约束
C. 一个表中最多只能有一个主键约束,可以有多个外键约束
D. 一个表中最多只能有一个外键约束,可以有多个主键约束

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