Logical Data Modeling - Entity (Row | Tuple | Member | Unit | Instance )
Table of Contents
1 - About
An entity is one thing which can be distinctly identified.
Example:
- A specific person
- A specific company
- A specific group of persons,
- A specific place,
- A specific event
It's a concept to abstractly group similar things.
2 - Articles Related
3 - Example of entity implementation by domain
An entity would be represented:
- In a database model, by a row where the Primary Key identifies a single instance
- in Mathematics, by a tuple.
- in Statistics, by a unit, an observation
- in Olap, by a member
- in a graph/network by a Node
- In code, by a a class (or struct).
- in a file system, by a file
4 - Properties
4.1 - Attribute
An attribute is a descriptive property or characteristic of an entity.
4.2 - Value
The value about an entity is expressed by a set of attribute-value pairs
<math> \begin{array}{rrr} att1 & \mapsto & Nico \\ att2 & \mapsto & Blue \\ att3 & \mapsto & 20 \\ att4 & \mapsto & Sunny \\ \end{array} </math>
4.3 - id
4.4 - Relationship
Entities are associated via relationship
5 - Type
5.1 - Permanent
A permanent entity is always part of the system, never entering or leaving.
5.2 - Temporary
A temporary entity is not always present in a system. It enters the system at some point in time, stays for a while, and then leaves again at some other point in time.
A transaction, a customer in a restaurant is an example of a temporary entity.
There is a constant flow of temporary entity.
5.3 - Active
An active entity (also known as dynamic model components) are entities that follow a process lifecycle (hence active and not passive).
Dimension are generally not an active component whereas a transaction is.
6 - Communication
Asynchronous communication between entities is based on signal and reception