Data Modeling - Relationships

The relationship is the core aspect of a relational database.

For instance, a many-to-one relationship is a link between two entities when one entity has a foreign key that references the primary key of another entity.

All relationships are implicitly bidirectional, meaning that they can interpreted in both directions.

When you have foreign key that NOT references the primary key of another table, the database has not enforced referential integrity.

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 :

• one-to-one
• optional on one side one-to-one
• …

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

A relationship is a description in a business document that has a verb that relates two entity types:

• has
• consists of
• uses

Pretend the recursive relation is between two tables, the real one and a virtual copy.

In this case, a manager table and an employee table.