Entity-Relationship Model

Captures Metadata in terms of:

• Entities
• Attributes
• Relationships

Graphical annotation:

• Entities labeled with rectangles
• attributes with circles
• relationships with a diamond
• double rectangle on existence dependencies entities
• dashed lines for weak attributes entity set
• Specialization:

Generalization:

We need role names whenever a component.

Four varieties of integrity constraints in an ER commonly expressed with graphical annotations:

• primary keys: identifies
• binary relationship types: (N:N), (N:1), (1:N), (1,1)
• existence dependencies: entities that needs another entities that exist (double on an entity set)
• general cardinality constraints: puts a lower and upper bound on number of relationships

EER Modelling

Attributes have attributes of their own

• Structured Attributes: Attributes have attributes of their own, multiple attributes
• Aggregation: the big rectangles - relationship can be aggregated to enable its relationships to be
• Specialization: entities that have their own subclass entities
• Generalization: Covers, entities that have a leas one of two or more other entity sets subsets.
• Disjointness: Overlaps,

Reification

Design Methodology

• When to introduce an attribute versus an entity set.
• When to introduce an entity set versus a relationship set.
• Choosing the arity of relationship sets.
• The use of extended features such as aggregation. ▶ Methodological considerations.

Attributes vs Entity Sets

• Maintain additional information on it
• Separate object
• Can multiple employees have diff phone numbers : Can several of its kind belong to a single entity
• Does it make sense to delete such an object
• Can it be missing from some of the entity set’s entities?
• Can it be shared by different entities?

Entities vs Relationship Sets

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