Powerdesigner tutorial series (1) Conceptual Data Models

This article from: http://www.cnblogs.com/yxonline/archive/2007/04/09/705479.html

Target:
This document describes the basic concepts of Data Model CDM in powerdesigner.

1. Conceptual Data Model Overview
Data models are the abstraction of data features in the real world. The data model must meet the following requirements:
1) Ability to simulate the real world more realistically
2) easy to understand
3) easy to implement by Computer

The conceptual data model is also called the information model. It is based on the entity-relationship (E-R) theory and expands this theory. It models information from the user's point of view and is mainly used for Conceptual Design of databases.

Generally, people first abstract the real world into a conceptual world, and then convert the conceptual world into a machine world. In other words, objective objects in the real world are first abstracted as entities and relationship. They do not depend on a specific computer system or a DBMS system, this model is what we call CDM, and then converts CDM to the data model supported by a DBMS on the computer. This model is the physical data model, that is, PDM.

CDM is a set of strictly defined model elements that accurately describe the system'sStatic, dynamic, and integrity constraints, IncludingData structure, data operations, and integrity constraintsThree parts.
1) Data structures are expressed as entities and attributes;
2) The data operation table is the insert, delete, modify, query, and other operations of records in the entity;
3) integrity constraints are expressed as integrity constraints of data (such as data types, checks, rules, etc.) and integrity constraints of reference between data (such as links and inheritance links );

Ii. Definitions of entities, attributes, and identifiers
Entity, also known as an instance, corresponds to "events" or "things" that can be different from other objects in the real world ". For example, each student in the school and each operation in the hospital.
Each entity can be used to describe a set of properties of entity features, which are called attributes. An entity is described by several attributes. For example, a student entity may consist of attributes such as student ID, name, gender, date of birth, department, and year of admission.

An entity set is a set of entities of the same type and nature. For example, a set of all students in a school can be defined as a "student" entity set, each entity in the "student" entity is subject to a student ID, name, gender, date of birth, department, and year of admission.

Entity type is a set of common properties of each entity in an entity set. For example, the entity type of "patient" is: Patient {outpatient number, name, gender, age, ID card No .............}. An object is an instance of the object type. When the meaning is clear, the object and object types are usually used interchangeably.

Each entity in an object type contains one or more attributes that uniquely identify it. These attributes are called identifiers of an object type. For example, a student ID is an identifier of a student entity type, "name", "Date of Birth", and "email address" form the "Citizen" entity type identifier.

Some object types can have several groups of attributes that act as identifiers. select one of these attributes as the primary identifier of the object type, and others as the secondary identifier.

3. entity, attribute, and identifier expression

To be continued!