PowerDesigner Tutorial Series (i) Conceptual data model

Target:
This paper mainly introduces the basic concept of CDM in PowerDesigner conceptual data model.
I. Overview of the conceptual data model
The data model is the abstraction of data features in the real world. The data model should meet three requirements:
1) able to realistically simulate the real world
2) Easy to understand
3) Convenient for computer implementation

The conceptual data model is also called the information model, which is based on the theory of entity-relation (entity-relationship, abbreviation e-r), and expands the theory. It models the information from the user's point of view and is mainly used in the conceptual design of the database.

Often people first abstract the real world into a conceptual world and then turn the conceptual world into a machine world. In other words, the objective object of the real world is abstracted into entity and relation (relationship), it does not depend on the specific computer system or a DBMS, this model is what we call CDM; The CDM is then converted to a data model supported by a DBMS on the computer, which is the physical data model, or PDM.

CDM is a set of strictly defined model elements that accurately describe the static, dynamic, and integrity constraints of the system, including data structure, data manipulation, and integrity constraints.
1) Data structures are expressed as entities and attributes;
2) Data operation is expressed as inserting, deleting, modifying and querying the records in the entity;
3) Integrity constraints are expressed as the integrity constraints (such as data types, checks, rules, etc.) of the data and referential integrity constraints (e.g., contact, inheritance, etc.) between data;


Ii. definition of entities, attributes and identifiers
Entities (entity), also known as instances, correspond to "events" or "things" in the real world that can be distinguished from other objects. For example, every student in the school, every surgery in the hospital.
Each entity has a set of properties that describe an entity's characteristics, called an attribute, and an entity is described by several attributes. If the student entity can be composed by the number, name, gender, date of birth, department, year of entry and other attributes.

An entity set is a collection of specific entities of the same type and of the same nature. For example, a collection of all students in a school can be defined as a "student" entity set, and each entity in the "student" entity has a character such as number, name, gender, date of birth, department, year of entry, etc.

An entity type is a collection of the common properties of each entity in the entity set, such as the "Patient" entity type: Patient {Outpatient number, name, gender, age, social Security number ... ｝。 An entity is an instance of an entity type, and, in the case of a clear meaning, the entity, entity type is usually used interchangeably.

Each entity in an entity type contains one or a set of properties that uniquely identify it, which is known as an identifier for the entity type (Identifier), such as an identifier for the student entity type, name, date of birth, and address, which together form the identifier for the citizen entity type.

Some entity types can have several sets of properties acting as identifiers, one of which is selected as the primary identifier of the entity type, and the other as the secondary identifier.

Iii. representation of entities, attributes and identifiers