数据库各种码与关系代数

本文主要是介绍数据库各种码与关系代数，对大家解决编程问题具有一定的参考价值，需要的程序猿们随着小编来一起学习吧！

Key

• K is a superkey of R if values for K are sufficient to identify a unique tuple
• K is a candidate key if superkey K is minimal
• Primary Key is the candidate key chosen by the database designer. We should choose the attribute set whose value rarely change

Foreign Key Constraint

• Foreign key constraint
  The value of a foreign key must be a value of the primary key of the referenced relation, or be null
• A relation can has zero, one, or more foreign keys
  advisor (s_id, i_id)
• A relation can reference itself.
  employee (ID, name, …, manager)
  manger is the ID of the employee’s manager

Referential integrity constraint

实体完整性：实体完整性是指关系的主关键字不能重复也不能取“空值"。

参照完整性：参照完整性是定义建立关系之间联系的主关键字与外部关键字引用的约束条件。

Relational Algebra（关系代数）

Select Operation

Project Operation

• The result is defined as the relation of k columns obtained by erasing the columns that are not listed
• Duplicate rows removed from result, since relations are sets
  

Generalized Projection

Composition of Relational Operations

Cartesian-Product Operation
![](https://www.www.zyiz.net/i/ll/?i=img_convert/7c0d2a115b206c76fcd39e281476b754.png

Join Operation

Natural Join Operation

Outer Join

1. Left Outer Join
2. Right Outer Join
3. Full Outer Join

Assignment Operation

Aggregate Operation

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