Relational Data Modeling - Relationship

1 - About

relationship in a relational model.

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

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).

3 - 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 :

but you can found other types as :

  • optional on one side one-to-one
  • Relational Data Modeling - Recursive Relationship

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

4 - Relationship table

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

Every table with a foreign key holds a relationship.

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