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关于候选键和超级键

[英]Regarding Candidate Keys and Superkeys

原文 2012-03-01 18:25:43 0 1 sql/ primary-key/ tuples

I have a quick question regarding candidate keys and superkeys. 我有一个关于候选键和超级键的快速问题。 Say you have two keys (a, b) where 'a' is a primary key and b is a candidate key. 假设您有两个键（a，b），其中“ a”是主键，b是候选键。 Would the combination of these two keys be a superkey ie. 这两个键的组合是否为超级键，即。 would (a,b) be a superkey? （a，b）会成为超级键吗？ Or would it be a candidate key. 还是将其作为候选密钥。 My assumption is that it would be a superkey because the definition of a candidate key states that it is a irreducible superkey and the combination of the two fields a and b could be reduced to either a or b. 我的假设是它将是一个超键，因为候选键的定义表明它是一个不可约的超键，并且两个字段a和b的组合可以减少为a或b。 Is this logic correct? 这个逻辑正确吗？ Or am I missing something here? 还是我在这里想念什么？ Thanks! 谢谢！

1 个解决方案

Would the combination of these two keys be a superkey ie. 这两个键的组合是否为超级键，即。 would (a,b) be a superkey? （a，b）会成为超级键吗？

Yes, it would still uniquely identify rows. 是的，它仍将唯一地标识行。

Or would it be a candidate key. 还是将其作为候选密钥。

No, it would no longer be minimal. 不，它不再是最小的。

My assumption is that it would be a superkey because the definition of a candidate key states that it is a irreducible superkey and the combination of the two fields a and b could be reduced to either a or b. 我的假设是它将是一个超键，因为候选键的定义表明它是一个不可约的超键，并且两个字段a和b的组合可以减少为a或b。 Is this logic correct? 这个逻辑正确吗？

Almost. 几乎。 Yes it would be a superkey, but not because it can be reduced. 是的，这将是一个超级键，但不是因为可以减少它。 It would be a superkey because it is unique. 这将是一个超级键，因为它是唯一的。

Every candidate key is superkey, but not every superkey is candidate key. 每个候选键都是超级键，但并非每个超级键都是候选键。 So {a} is both candidate and superkey, {b} is both candidate and superkey and {a, b} is just superkey. 因此{a}既是候选者又是超键， {b}既是候选者又是超键，而{a, b}只是超键。