# ER, EER Diagram Notation: Entities, Participation, Cardinality and Constraints for AFCAT

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## Constraints - Participation

• Total Participation - entity X has total participation in Relationship Z, meaning that every instance of X takes part in AT LEAST one relationship. (i.e.. there are no members of X that do not participate in the relationship.

Example: X is Customer, Y is Product, and Z is a ‘Purchases’ relationship. The figure below indicates the requirement that every customer purchases a product.

• Partial Participation - entity Y has partial participation in Relationship Z, meaning that only some instances of Y take part in the relationship.

Example: X is Customer, Y is Product, and Z is a ‘Purchases’ relationship. The figure below indicates the requirement that not every product is purchases by a customer. Some products may not be purchased at all.

## Constraints - Cardinality

• 1: N – One Customer buys many products, each product is purchased by only one customer.
• N: 1 - Each customer buys at most one product, each product can be purchased by many customers.
• 1: 1 – Each customer purchases at most one product, each product is purchased by only one customer.
• M: N – Each customer purchases many products, each product is purchased by many customers.

## Specialization/Generalization

• Each subclass inherits all relationships and attributes from the super-class.

Constraints on Specialization/Generalization

• Total Specialization – Every member of the super-class must belong to at least one subclass. For example, any book that is not a text book, or a novel can fit into the “Other” category.
• Partial Specialization – each member of the super-class may not belong to one of the subclasses. For example, a book on poetry may be neither a text book, a novel or a biography.

## Disjointness Constraint

• Disjoint – every member of the super-class can belong to at most one of the subclasses. For example, an Animal cannot be a lion and a horse, it must be either a lion, a horse, or a dog.

Overlapping – every member of the super-class can belong to more than one of the subclasses. For example, a book can be a text book, but also a poetry book at the same time.

Multiple Inheritance – a subclass participates in more than one subclass/super-class relationship, and inherits attributes and relationships from more than one super-class. For example, the subclass Mermaid participates in two subclass/super-class relationships, it inherits attributes and relationships of Animals, as well as attributes and relationships of Humans.

Union – a subclass/super-class relationship can have more than one super-class, and the subclass inherits from at most one of the super-classes (i.e.. the subclass purchase will inherit the relationships and attributes associated with either service or product, but not both) . Each super class may have different primary keys, or the same primary key. All members of the super-classes are not members of the super-class. For example, a purchase can be a product, or a service, but not both. And all products and services are not purchases.