ch2RelationalModel.ppt

Chapter 2: Relational Model Structure of Relational Databases Database Schema Keys Schema Diagrams Relational Query Languages Relational Operations Chapter 2: Relational Model 教学目的： 熟悉关系数据模型 教学重点： 简单属性、多值属性、复合属性 关系、关系模式、码等概念 关系模式图 教学难点： 超码、候选码、主码、外码等概念 Structure of Relational Databases Example of a Relation Attributes Attribute Domain The special value null is a member of every domain Attribute values are (normally) required to be atomic; that is, indivisible multivalued attribute values are not atomic E.g. composite attribute values are not atomic E.g. Exercises Which attribute is characterized in relational database system? A. Simple attribute B. Composite attribute C. Multi-valued attribute D. Derived attribute Relational Model “关系”的数学定义 设集合 Color = { 红，绿，黄 } 设集合 Fruit = { 苹果，香蕉，雪梨} 这两个集合的笛卡尔积 Color X Fruit 是这样的一个集合： {（红，苹果）、（红，香蕉）、（红，雪梨）、 （绿，苹果）、（绿，香蕉）、（绿，雪梨）、 （黄，苹果）、（黄，香蕉）、（黄，雪梨）} 集合Color X Fruit的任意子集，叫做“关系”，例如： {（红，苹果）、 （绿，苹果）、（黄，香蕉）、（黄，雪梨）} Definition: Cartesian-Product of Sets D1 x D2 x … x Dn = { (a1, a2, …, an) | ai ? Di } Definition: a relation r is a subset of D1 x D2 x … x Dn Relation Example: Suppose there exists three sets dept_name = {Biology, History, Physics ……} building = {Watson, Taylor, Painter ……} budget = {10000, 20000, 30000……}the set department = { (Biology, Watson, 90000),  (Finance, Painter, 120000), (History, Painter, 50000) } is a relation over dept_name x building x customer_budget Relation and Table are equivalent department = { (Biology, Watson, 90000),  (Finance, Painter, 120000), (History, Painter, 50000) } Databases Schema Relation Schema R = (A1, A2, …, An ) is a relation schema We use names beginning with an uppercase letter for relation schemas. Example: Instructor_schema = (ID, name, dept_name,