Entity Relationship Modeling

Entity Relationship Modeling gives an insight to the LN application databases and the way in which they are interrelated.

Entity Relationship Modeling is composed of two main building blocks:

  • Entity types
  • Entity relationships

These building blocks are interrelated and used in entity relationship diagrams to show the relationships between the permanent storage components. Together with the other components listed later, they let you illustrate the diversity of relationships between the different databases. A group of related diagrams make up an entity relationship model.

  • Entity type
    A person, place, thing, or concept that you want to record information about. An entity type is a group of entities with common attributes and can be part of a diagram, such as Trucks.
  • Entity
    A single occurrence of an entity type; a fact relevant to the company, and about which information is permanently stored. For example: Truck-A and Truck-B.

Three kinds of entities exist:

  • Logical entity
    Entities that have a meaning to the real world and are comprised of one or more physical entities; they are defined on a higher abstraction level.
  • Physical entity
    Database tables of the LN application.
  • Associative entity
    An entity used to link other entities. An associative entity type is only used when there is a many-to-many relationship between two entity types.
Example

There could be an M:N relationship between EMPLOYEE and TRUCK regarding maintenance.

This relationship does not show which EMPLOYEE maintains which TRUCK.

There are also other attributes to be considered, such as time spent, part no. and so on.

As a result of the ambiguity regarding the Many-to-many relationship, the associative entity type MAINTENANCE can be defined. (See this diagram).

Associative entity type

Associative entity type

  • Attribute
    A fact or non-decomposable piece of information describing an entity type; for example, Number of wheels.
  • Attribute value
    The value of an attribute; for example, 4 wheels.
  • Relationship
    A reason of relevance for associating entities of one or two entity types.
  • Cardinality
    A specification of the number of possible entities for each entity type of a pairing.

The three types of cardinality are:

  • One-to-One (1:1)
    A one-to-one relationship. Only one instance of entity type B can be associated with only one instance of entity type A.
  • One-to-Many (1:N)
    A one-to-many relationship. Multiple instances of entity type B can be associated with only one instance of entity type A.
  • Many-to-Many (M:N)
    A many-to-many relationship. Multiple instances of entity type B can be associated with multiple instances of entity type A and vice-versa.

Within Entity Relationship Modeling, there is no real difference between logical and physical entities.

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