第一范式、第二范式、第三范式 第一范式 如果一个关系模式R的所有属性都是不可分的基本数据项,则R1NF(即R符合第一范式)。 两点: 一、每个字段都只能存放单一值 课程有两个值,不符合第一范式,可改为如下 二、每笔记录都要能利用一个惟一的主键来加以识别
第一范式、第二范式、第三范式
第一范式
如果一个关系模式R的所有属性都是不可分的基本数据项,则R∈1NF(即R符合第一范式)。
两点:
一、每个字段都只能存放单一值
课程有两个值,不符合第一范式,可改为如下
二、每笔记录都要能利用一个惟一的主键来加以识别
这里出现了重复组,同样不满足第一范式,因为缺乏唯一标识码,可改为
第二范式
若关系模式R∈1NF(即R符合第一范式),并且每一个非主属性都完全依赖于R的码,则R∈2NF(即R符合第二范式)。
这里表的码为(学号,课程), 即知道这两项可以确定系名、宿舍、分数,或者是这三项依赖于前两项,可知
分数完全依赖(学号,课程)系名部分依赖(学号,课程),即知道学号或者课程就能确定系名.宿舍楼部分依赖(学号,课程),即知道学号或者课程就能确定宿舍楼.
由于非主属性系名,宿舍楼不完全依赖与码,不符合第二范式,可改为
表1
第三范式
若关系模式R∈3NF(即R符合第三范式),则每一个非主属性既不部分依赖于码也不传递依赖于码。
上面的表2不符合第三范式,这是因为你知道了系名,同样也就知道了宿舍楼,称宿舍楼传递依赖于码(学号),可分解为

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