1. Overview of Entity‐Relationship Model
Entity‐Relationship Model is a detailed, logical representation of the data for an organization or for a business area.
What are the entities and relationships in the enterprise?
What information about these entities and their relationships should we store in the database?
What are the integrity constraints or business rules that holds?
The model must be as ‘open’ as possible and not tied to any technology or to any particular business methodology.
Introduced in 1976 by Peter Chen.
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2. Supertypes and Subtypes
Subtype: A subgrouping of the entities in an entity type that has attributes distinct from those in other subgroupings
Supertype: A generic entity type that has a relationship with one or more subtypes
Attribute Inheritance:
Subtype entities inherit values of all attributes of the supertype
An instance of a subtype is also an instance of the supertype
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3. EER Notations
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4. EER Notations (Microsoft Visio)
Different modeling tools may have different notation for the same modeling constructs
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5. Supertype/Subtype Example
All employee subtypes will have employee number, name, address, and date‐ hired
Each employee subtype will also have its own attributes
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6. Relationships and Subtypes
Relationships at the supertype level indicate that all subtypes will participate in the relationship
The instances of a subtype may participate in a relationship unique to that subtype. In this situation, the relationship is shown at the subtype level
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7. Another Example: Hospital
Both outpatients and resident patients are cared for by a responsible physician
Only resident patients are assigned to a bed
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8. Word of caution! When to use sub/super‐type relationships?
There are attributes that apply to some (but not all) instances of an entity type.
Some (again not all) instances of an entity type participate in a relationship
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9. Generalization and Specialization
Generalization: The process of defining a more general entity type (supertype) from a set of more specialized entity types (subtypes) and forming supertype/subtype relationships.
BOTTOM‐UP
Specialization: The process of defining one or more specialized entity types (subtypes) of the supertype.
TOP‐DOWN
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10. Example of Generalization
Three entity types: CAR, TRUCK, and MOTORCYCLE
All these types of vehicles have common attributes
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11. Example of Generalization (2)
Generalization to VEHICLE supertype
So we put the shared attributes in a supertype
Note: no subtype for motorcycle, since it has no unique attributes
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12. Example of Specialization
Only applies to manufactured parts
Applies only to purchased parts
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13. Example of Specialization (2)
Created two subtypes
Note: multivalued attribute was replaced by an associative entity relationship to another entity
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14. Constraints in Supertype/Subtype
Completeness Constraints: Whether an instance of a supertype must also be a member of at least one subtype.
No further sub‐type(s) of the super‐type is possible.
Yes: Total Specialization Rule (double line)
No: Partial Specialization Rule (single line)
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15. Example of Completeness Constraint
Total specialization rule
A patient must be either an outpatient or a resident patient
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16. Example of Completeness Constraint
Partial specialization rule
A vehicle could be a car, a truck, or nothing.
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17. Constraints in Supertype/Subtype
Disjointness Constraints: Whether an instance of a supertype may simultaneously be a member of two (or more) subtypes
Disjoint Rule: An instance of the supertype can be only ONE of the subtypes
Overlap Rule: An instance of the supertype could be more than one of the subtypes
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18. Example of Disjointness Constraint
Disjoint rule
A patient can either be outpatient or resident, but not both
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19. Example of Disjointness Constraint
Overlap rule
A part may be both purchased and manufactured
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20. Constraints in Supertype/Subtype
Subtype Discriminator: An attribute of the supertype whose values determine the target subtype(s)
Disjoint – a simple attribute with alternative values to indicate the possible subtypes
Overlapping – a composite attribute whose subparts pertain to different subtypes. Each subpart contains a boolean value to indicate whether or not the instance belongs to the associated subtype
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21. Subtype Discriminator (Disjoint Rule)
A simple attribute with different possible values indicating the subtype
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22. Subtype Discriminator (Overlap Rule)
A composite attribute with sub‐attributes indicating “yes” or “no” to determine whether it is of each subtype
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23. Supertype/Subtype Hierarchy
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24. Entity Clusters
Problem: EER diagrams are difficult to read when there are too many entities and relationships
Solution: Group entities and relationships into clusters
Entity cluster: Set of one or more entity types and associated relationships grouped into a single abstract entity type ll
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25. Pine Valley Furniture Company
Related groups of entities could become clusters
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26. EERD of PVF Entity Clusters
More readable, isn’t it?
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27. Manufacturing Entity Cluster
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28. Conclusion Summary Essential Reading What is Next?
Basic business rules are data names and definitions
Data modeling notations frequently used today is the entity‐relationship data model.
E‐R model constructs: entities, entity types, relationships, relationship types, attributes, identifiers, cardinality constraints.
Essential Reading
Modern Database Management (8th Ed.), Chapter 3
What is Next?
Relational Model
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