Data Description Reality World -> Cognition Abstract -> Information World (Concept Model) - (Conversion) -> Machine World (DBMS) Information World Basic Concept: 1. Entity: objectively existently and can distinguishably referred to as entities. 2. Entity Set: Collection of similar entities with the same properties 3. Attribute: An entity has a particular characteristic called attribute 4. Entity identifier: Unique identity entity attribute or property set, or referred to as the basic concept of the code machine world. Field: Tagnate 2 of the entity property. Record: Order collection of fields is called record 3. File: Collection of the same class record is called file 4. Key code: Unique identity field or field set information World Machine World Entity Record Property field entity set file entity identifier key code data model concept 1. Conceptual model (information model): From the user's point of view, the demand concept is simple, clear, easy to understand, is the first layer of abstraction 2 for the real world. Data model (mesh, layered, relationship): From a computer point of view, requires strict syntax and semantic languages to strictly formulate data on data, so that it can be transformed into a computer to understand Three elements of the format data model: 1. Data structure 2. Data operation 3. Integrity Constraint Concept Model - Entity Relationship Model ER Concept Model The most commonly used entity-relationship approach rectangle: Index: The entity is elliptical: indicates that the attribute rhombus: Contact line: Contact type and its design Types are connected in line with a line of linear ends.
