Ch4 High-Level Database Models.ppt

Database Principles Chapter4High-Level Database Models The Entity/Relationship Model Design Principles Constraints in E/R Model Weak Entity Sets From E/R Diagrams to Relational Designs 4.1 The Entity/Relationship Model The concept model is represented by Entity-Relationship（E-R）diagrams. The E/R model Describes kinds of data and how they connected. Can be converted to relational model later. Conceptual Model Entity（实体） : The things existent objectively and distinguished each from other possibly are called Entity. It can be the actual things;can be the abstract concepts or relationship Examples：Employee,Student,Department Attributes（属性）: The properties of a entity are called attributes of that entity. Examples: Entity Student can has attributes Number of student,name ,age, berth date and etc. Conceptual Model(Cont.) Key（码） The attribute that can identify the entity uniquely is called the Key of that Entity Domain（域） The scope in which the attribute’s value falls. Conceptual Model(Cont.) Entity Type（实体型） Representation of a class of entities with the same attributes and the same properties; The entity name and the attribute’s name are used to represent entity type. Example: Student ( Num, Name,Sex,Age, Berth Date,Dept) is student entity type Entity Set（实体集） The set of entities with the same entity type; Example: All students Conceptual Model(Cont.) Relationships（联系） A relationship connects two or more entity sets. It is represented by a diamond, with lines to each of the entity sets involved. E/R Diagrams In an entity-relationship diagram: Entity set = rectangle. Attribute = oval, with a line to the rectangle representing its entity set. Example Entity set Stars has two attributes, name and address. Each star has values for these two attributes. Relationships A relationship connects two or more entity sets. It is represented by a diamond, with lines to each of the entity sets involved. Example Relationship Set E/R diagram desc