Tags:conceptdatabaseerdrelationshipcardinalitiescardinalities Status:🟩

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ER Modeling Relationship Cardinalities

Summary

Cardinalities in ER diagrams define the numerical relationships between entities, indicating how many instances of one entity can or must relate to instances of another. They describe the minimum and maximum number of entity occurrences involved in the relationship. Relationships always have cardinalities.

Example:

• A student is enrolled in 1 or more courses. So at least 1 course.
• A student can be assigned to maximum 1 master thesis.
• An employee can manage 0 or many projects. A project must be managed my exactly 1 employee.

Details

Maximum 1

Zero or many professors works in zero or one departments. (Maximum 1 department per professor).

Maximum 1 in both directions

Zero or one professor works in zero or one department.

Exactly 1

Zero or many professors works in exactly 1 department. (all professors works just 1 department).

Exactly 1 in both directions

One professor works in 1 department.

Minimum 1 - Maximum N

One or many professors works in at least 1 or many departments.