Objects in a LDM

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The following items are available in a LDM:

 

Object

Tool

Description

Domain

-

A set of values valid for a certain piece of data, these can be used as the basis for Attributes.

Entity

An "object" in the abstract sense of the word. It's data that are of importance to the application, eg: person, book, invoice. In the physical diagram, this will generate a table.

Attribute

-

A placeholder for a value (piece of data) in an entity.

Identifier

-

An attribute or combination of attributes that uniquely identifies an instance of the entity.

Relationship

A "connection" or "relationship" between entities. One entity someone depends on another. Depending on the available relationship options, this will generate a physical Foreign Key Constraint with or without an intermediate table.

 

See also: Objects in any Diagram.

 

 

Domain

Domains make it easier to identify the types of information in your model, assigning domains to attributes help you standardize data characteristics for entity attributes in different entities.

 

Just like an "attribute" of an Entity, each Domain has a datatype. In addition to that, each domain can have a set of allowed values or have a minimum and maximum value defined, specified maximum length or precision and a scale. This makes the possible values in a "domain" a sub-set of the available values for a given datatype. Domains can be used as the "datatype" of an attribute instead of a raw datatype.

 

A Domain can also have a "default value" in those cases no value is used.

 

Domains are not visible in the diagram itself, but are visible in the Diagram Explorer.

 

See also:

Creating a Domain
Domains to a PDM
Datatypes in a LDM

 

Entity

Each entity represents a real world "object", in the abstract sense of the word. An entity contains data about real world concepts, eg: Book, Invoice, Car, Person. Each entity should have one or more attributes describing the entity, eg: First Name, Last Name, Address (of entity Person). Attributes can be optional (= no value required) or required.

 

In addition to normal attributes, an entity should have an "identifier" which value uniquely identifies an instance of the entity. An "identifier" can be a single attribute (eg: EmployeeNumber) or a combination of attributes. Each of the identifier attributes will be required to have a value.

 

Entities can have relationships with other entities, eg: a Person has a Car.

 

See also:

Creating an Entity
Entities to a PDM

 

Relationship

One entity can have something to do with another entity, eg: a Person has a Car. This is called a "relationship" between entities.

 

A relationship can have many different forms, called "roles". In "a Person has a Car", there are two roles: "has" and "belongs to". Each Car belongs to a Person and each Person can have a car.

 

Besides roles, there's "cardinality", in our example: "a Person has 1 Car" is different from "a Person can have 1 or more Cars" which is different from "a Person can have zero or more Cars". These different cardinalities are available for each relationship.

 

See also:

Creating a Relationship
Relationships to a PDM
Relationship Types

 

 

Next: Datatypes in a LDM