Maximum cardinality is the largest number of target-entity instances that one source-entity instance can be associated with. In a directional relationship from A to B, it describes how many B instances one A instance may have. Crow’s Foot notation places this symbol on the relationship line near the target entity. section 5.5
Read maximum cardinality from simple relationship statements:
- “A user has zero, one, or more accounts” gives the user-to-account relationship a maximum cardinality of many. section 5.5
- “An account is associated with one and only one user” gives the account-to-user relationship a maximum cardinality of one. section 5.5
- A one-to-one relationship has a maximum cardinality of one in both directions; its minimum cardinalities may still be zero or one. section 5.6
Department and manager
If each department has one manager and each manager works for one department, both directions have maximum cardinality one. Relaxing the department-to-manager minimum to zero can make foreign-key implementation possible without changing that maximum. section 5.6
