why must a primary key be minimal?

Question

A primary key is an attribute or set of attributes that uniquely identifies a row in a table. But a primary key has not only to be unique, but also minimal. Why is that necessary?

Answer 1

识别最小密钥很重要的原因是为了确保这些密钥的依赖性没有冗余.冗余可能导致异常和不正确的结果.

最小化本质上是一个语义问题而不是纯粹的结构特征,因此数据库实现不一定需要它.例如,SQL允许您在任何超级密钥上创建"PRIMARY KEY",这可能不是不可简化的超级密钥.

最小化与存储大小无关,因为最小化意味着不可简化,并不意味着最小.