Home Database Mysql Tutorial 菜鸟学数据库(四)超键、候选键、主键、外键

菜鸟学数据库(四)超键、候选键、主键、外键

Jun 07, 2016 pm 03:11 PM
primary key foreign key database rookie

这些年的一些经历告诉我,很多初学者搞不清超键、候选键等,被 数据库中的 各种键搞的一头雾水。下面就跟大家一起聊聊数据库中的那些键。 首先看看各种键的定义: 超键( super key):在关系中能唯一标识元组的属性集称为关系模式的超键 候选键 (candidate key

这些年的一些经历告诉我,很多初学者搞不清超键、候选键等,被数据库中的各种键搞的一头雾水。下面就跟大家一起聊聊数据库中的那些键。


首先看看各种键的定义:


超键(super key):在关系中能唯一标识元组的属性集称为关系模式的超键

候选键(candidate key):不含有多余属性的超键称为候选键

主键(primary key):用户选作元组标识的一个候选键程序主键

外键(foreign key)如果关系模式R1中的某属性集不是R1的主键,而是另一个关系R2的主键则该属性集是关系模式R1的外键。



结合实例的具体解释:


假设有如下两个表:

学生(学号,姓名,性别,身份证号,教师编号)

教师(教师编号,姓名,工资)


超键:

由超键的定义可知,学生表中含有学号或者身份证号的任意组合都为此表的超键。如:(学号)、(学号,姓名)、(身份证号,性别)等。


候选键:

候选键属于超键,它是最小的超键,就是说如果再去掉候选键中的任何一个属性它就不再是超键了。学生表中的候选键为:(学号)、(身份证号)。


主键:

主键就是候选键里面的一个,是人为规定的,例如学生表中,我们通常会让“学号”做主键,教师表中让“教师编号”做主键。


外键:

外键比较简单,学生表中的外键就是“教师编号”。外键主要是用来描述两个表的关系。


其实这些键还是比较容易区分的,只是初学者一下子看到好几个名字相似的概念,容易自己把自己搞晕,自己给自己制造困难。多读两遍定义,然后结合实例很容易就能把它们区分开。希望这篇文章可以给一些初学者一点点的帮助。



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