数据库课件lec2TheEntityRelationshipModel幻灯片.pptVIP

* 6 The Entity-Relationship Model Jianlin Feng School of Software SUN YAT-SEN UNIVERSITY courtesy of Joe Hellerstein for some slides Data Models DBMS models real world. Data Model links user’s view of the world and bits stored in computer. We will ground ourselves in the Relational model But use the Entity-Relationship (ER) model as a middle ground for design Steps in Database Design Requirements Analysis user needs; what must database do? Conceptual (Database) Design high level (or semantic) description from users’ daily view. often done with the ER model Logical (Database) Design translate ER into DBMS data model (e.g., Relational Model) Schema Refinement consistency, normalization Physical (Database) Design - indexes, disk layout Security Design - who accesses what, and how Conceptual Design What are the entities and relationships in the enterprise? What information about these entities and relationships should we store in the database? What are the integrity constraints or business rules that hold? A database `schema’ in the ER Model can be represented pictorially (ER diagrams). Can map an ER diagram into a relational schema. ER Model Basics Entity: Real-world object distinguishable from other objects. An entity is described (in DB) using a set of attributes. Entity Set: A collection of similar entities. E.g., all employees. All entities in an entity set have the same attributes. Each entity set has a key (underlined). Each attribute has a domain. Keys A key is a minimal set of attributes whose values uniquely identify an entity in some entity set. For each entity set, we choose a key. Candidate key An entity set can have more than one key. Primary key We designate one of the candidate keys as the primary key. Representation of an Entity Set Entity set Represented by a rectangle. Attribute: Represented by an oval. Key: Each attribute in the primary key is underlined. Employees ssn name lot ER Model Basics (Cont.) Relationship: Association among