数据库复习③

2016年6月15日

15:24

Main ER Model ER模型

1. ER模型中的约束constraints有哪些？

　　 Single value constraints 单值约束

　　　　EX： a person can have only one father.

　　Referential integrity constraints 引用完整性约束

　　　　EX： if you work for a company, it must exist in the database

　　Domain constraints 域约束

　　　　EX： peoples’ ages are between 0 and 150.

2.ER图中的各种键Keys概念

　　super key 超键

　　　　A super key of an entity set is a set of one or more attributes whose values uniquely determine each entity

　　　　在关系中能唯一标识元组的属性集称为关系模式的超键

　　candidate key 候选键

　　　　A candidate key of an entity set is a minimal super key

　　　　不含有多余属性的超键称为候选键

　　　　举例：Customer_id is candidate key of customer

　　primary key 主键

　　　　Although several candidate keys may exist, one of the candidate keys is selected to be the primary key

　　　　用户选作元组标识的一个候选键程序主键

　　Foreign key 外键

　　　　完整性约束的一种，要求该属性在另一表的某个key中出现。

3.弱实体集Weak Entity Sets

　　From https://en.wikipedia.org/wiki/Weak_entity

　　In a relational database, a weak entity is an entity that cannot be uniquely identified by its attributes alone; therefore, it must use a foreign key in conjunction with its attributes to create a primary key. The foreign key is typically a primary key of an entity it is related to.

4.ER模型的原则