Entity Relationship Modeling
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 that are 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
andTruck-B
.
Three types 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 that is 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.
This diagram shows the 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:
-
- 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.