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

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 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
• In the web/network, by a resource (more specifically a web page)

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

The value about an entity is expressed by a set of attribute-value pairs

<math> \begin{array}{rrr} att1 & \mapsto & Foo \\ att2 & \mapsto & Blue \\ att3 & \mapsto & 20 \\ att4 & \mapsto & Sunny \\ \end{array} </math>

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

Entities are associated via relationship

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

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.

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.

Asynchronous communication between entities is based on signal and reception