Reference 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

Goal:
This paper mainly introduces PowerDesigner conceptual data Model and entity and attribute creation.

A new concept data model
1) Select File-->new, Pop up the dialog box as shown, and select the CDM model (i.e. conceptual data model) to build the model.


2) Complete the creation of the conceptual data model. The following illustration shows a brief description of the current workspace. (More details later)


3) Select the new CDM model, right-click, select the "Properties" property item in the popup menu, and pop up the dialog as shown in the image. The "General" tab allows you to enter the name, code, description, creator, version, and default chart of the model you have built. In the "Notes" tab, you can enter the relevant description and description information. Of course there are more tags, you can click on the "more>>" button, here is no longer explained in detail.


Ii. creation of new entities
1) in the CDM's graphics window, click the Entity tool on the Tools tab, click the Space in the graphics window, and a solid symbol appears where you clicked. Click the Pointer tool or right-click the mouse to release the Entitiy tool. As shown in the figure

2) Double-click the entity symbol you just created, open the following icon window, where you can enter the name, code, description, etc. of the entity in the "General" tab of this window.


Third, add entity properties
1) You can add properties to the "Attribute" tab of the above window, as shown in the following figure.


Attention:
The features "Add attribute" and "reuse existing data items" in data items are related to the unique code and allow reuse options for data item in the model.
The P column indicates whether the property is the primary identifier, and the D column indicates whether the property is displayed in the graphics window; The M column indicates whether the property is mandatory, that is, whether the column is a null value.

If an entity property is mandatory, then this property must be assigned in each record and cannot be empty.

2) in the window shown above, click the Insert Properties button and the Properties dialog box appears, as shown in the following figure.

Note: This involves the concept of a domain, a standard data structure that can be applied to the properties of a data item or entity. In the following tutorials, separate chapters will be explained in detail.