ch2ER模型.ppt

Chapter 2: Entity-Relationship Model第二章：实体-联系模型 Entity Sets实体集 A database can be modeled as:数据库可由下列内容模型化 a collection of entities,实体集合 relationship among entities.实体间的关系 An entity is a “thing” or “object” in the real world that is distinguishable from all other objects.实体是现实世界中可区别于其他对象的“事件”或“物体”。 Example: specific person, company, event, plant例如：指定的人、公司、事件、工厂 An entity set is a set of entities of the same type that share the same properties.实体集是拥有相同特性的同类型实体的集合 Example: set of all persons, companies, trees, holidays例如所有人、公司、树、节假日的集合 The extension of the entity set——the individual entities that constitute a set 组成实体集的各实体称为实体集的外延 Entity Sets customer and loan实体集：客户和贷款 customer-id customer- customer- customer- loan- amount name street city number Relationship Sets联系集 A relationship is an association among several entities联系是若干实体间的关联Example:例如 Hayes depositor A-102 customer entity relationship set account entity 姓名（客户实体） 储户（联系集） A-102（帐目实体） A relationship set is a mathematical relation among n ? 2 entities, each taken from entity sets联系集是两个以上实体间的数学关系（所有实体来自实体集） {(e1, e2, … en) | e1 ? E1, e2 ? E2, …, en ? En}where (e1, e2, …, en) is a relationship其中(e1, e2, …, en) 是关系 Example:例如： (Hayes, A-102) ? depositor Relationship Set borrower Relationship Sets (Cont.)联系集（续） The association between entity sets is referred to as participation ;that is, the entity sets E1 ,E2, …,En in participate in relationship R. 参与 A relationship instance in an E-R schema represents that an association exists between the named entities in the real-world enterprise that is being modeled. The function that an entity plays in a relationship is called that entity’s role. 角色 A relationship may also have descriptive attributes. 联系也可能具有描述性属性。 Binary relationship sets 二元联系集 Nonbinary relationship sets,such as ternary relationship sets