Relationships

The important part of a relational database is the relationships between entities. A relationship is shown using a line (solid or dashed) between two entities. At both ends, the cardinality and modality (multiplicity) are shown.

Cardinality and Modality

The cardinality and modality refers to the number of rows an entity can have in the relationship. Cardinality refers to the maximum while modality refers to the minimum number of times an instance in one entity can be associated with an instance in the related entity.

The cardinality symbol is placed on the outside end of the relationship line (closest to the entity). It can either be a perpendicular line, indicating a cardinality of 1; or a foot with three toes, indicating a cardinality of many.

The modality symbol is placed next to the cardinality. It can either be a perpendicular line, indicating a modality of 1; or circle (zero), indicating a modality of zero.

So, we have these four instances:

• One and One only (1)

• One or Many (1..*)

• Zero or Many (0..*)

• Zero or One (0..1)

Let’s put it all together:

Types of Relationships

Above you can see that the relationship between Entity3 and Entity4 is dashed. This means that the relationship is a weak relationship – very much like the relationship with your ex.

There are types of relationships in ERDs:

• Weak (Non-identifying) Relationship: Primary Key of child entity doesn’t contain Primary Key of parent entity. Indicated using a dashed line.
• Strong (Identifying) Relationship Primary Key of child entity contains Primary Key of parent entity. Indicated using a solid line.