Logical Data Modeling - Entity (Row | Tuple | Member | Unit | Instance )

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.

An entity is a single occurrence of an entity set, it's also known as an instance

Example of entity implementation by domain

An entity would be represented:

Properties

Attribute

An attribute is a descriptive property or characteristic of an entity.

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>

id

An id is attribute that describes the identity of an entity.

Relationship

Entities are associated via relationship

Type

Permanent

A permanent entity is always part of the system, never entering or leaving.

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.

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.

Communication

Asynchronous communication between entities is based on signal and reception


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