数据库笔记4--关系代数(哈工大)

缺乏、安全感 2022-12-08 12:48 208阅读 0赞

# 数据库笔记4(哈工大) #

关系运算 = 关系代数（集合运算（并交差））+关系演算（元组演算（与或非）+域演算）  
1、关系代数是基于集合的，也就是说，不能有重复元素，元素也没有顺序。  
2、并相关性：满足并相关性才能进行关系运算，就是要满足： 1属性个数相同 2属性都是取自同一个域。  
注意，并操作是针对元组。也就是所谓的 “行”  
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注意区分**积**和**连接**操作  
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注意,投影是操作“列”  
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连接：等值连接可以保留自身更名后再自身连接的相同属性，自然连接会删除  
[可以理解为：自然连接就是除去重复属性的等值连接][Link 1]  
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除操作：可以理解为笛卡尔积的逆操作  
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### 外连接： ###

> 定义 ：两个关系R 与S 进行连接时，如果关系R( 或S) 中的元组在S( 或R)中找不到相匹配的元组，则为了避免该元组信息丢失，从而将该元组与S( 或R)中假定存在的全为空值的元组形成连接，放置在结果关系中，这种连接称之为外连接(Outer Join)

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左右全外连接  
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*  小结

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[Link 1]: https://blog.csdn.net/weixin_43889841/article/details/90669055
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