## About

This page is about relationship in a relational model.

In a relational model, as a relation (generally a table) can model:

- an entity
- or a relationship

Therefore, a relationship between two relations is not always a relations between two entities.

cardinality define the type of relationships between two relations.

Because most of database only support one-to-many relationships, it is necessary to implement other relationships physically via a third junction table (association table).

## Type

Mapping: Information about the number of entities in each entity set which is allowed in a relationship set is indicated by specifying 1, m, % in the diagram

In a data model, two main relationship are used :

- one-to-many (or many-to-one)
- and many-to-many

but you can found other types as :

- optional on one side one-to-one
- …

To describe precisely the type of relationship in a data model, you use the cardinality.

## Relationship table

The following tables are the most common tables that holds a relationship:

- Relational Data Modeling - Association Table (Bridge, Cross) (many-to-many)
- Dimensional Data Modeling - Fact Table (one-to-many)
- Dimensional Data Modeling - Hierarchy (one-to-many)

Every table with a foreign key holds a relationship.