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Data & Analytics · Intermediate

Optionality

Whether participation in a relationship is mandatory (minimum one) or optional (minimum zero).

Definition

Optionality, also called modality, complements cardinality by stating the minimum number of related instances. A mandatory relationship usually becomes a NOT NULL foreign key, while an optional one allows NULLs.

Example

An Order must have a Customer (mandatory), but a Customer may have zero Orders (optional).

Exam tip

Mandatory participation on the child side usually means a NOT NULL foreign key.

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