候选键SQL
[英]Candidate keys SQL
问题描述
hi am recently learning about SQL and keys. 
嗨，我最近在学习有关SQL和键的知识。
i am wondering if there is n attributes to a relation table how many possible Candidate keys would there be? 我想知道关系表是否有n个属性，可能有多少个候选键？
1 个解决方案
解决方案1
0 2016-03-21 10:00:17
Each subset of the attributes of the relations can be a candidate key. 关系的属性的每个子集都可以是候选键。 So the number is equal to the number of the subsets of a set of n elements, that is 2 n 因此，该数量等于一组n个元素的子集的数量，即2 n
Edited 已编辑
The original answer was 2 n -1, but it significant also to have the empty (or niladic) key. 最初的答案是2 n -1，但是拥有空（或尼拉迪克）键也很重要。 A key of such kind implies that the relation has only one tuple. 这种键意味着该关系只有一个元组。 Thanks to @ErwinSmout for the clarification. 感谢@ErwinSmout的澄清。
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