IS6125 Database Analysis and Design Lecture 7: Enhanced Entity-Relationship (EER) Modeling Rob Gleasure R.Gleasure@ucc.ie www.robgleasure.com

IS6125 Today’s session Enhanced Entity-Relationship Modelling An exercise

Example from Lecture 4: BigArt Galleries Galleries keep information about artists, their name (which is unique), birthplace, age, and one or more styles of art. For each piece of artwork, the artist, the year it was made, its unique title, its type of art (e.g., painting, lithograph, sculpture, photograph), and its price must be stored. Pieces of artwork are also classified into groups of various kinds, for example, portraits, still lifes, etc., and assigned a curator. A given piece may belong to more than one group. Each group is identified by a name (like those just given) that describes the group. Finally, galleries keep information about customers. For each customer, galleries keep that person’s unique name, address, total amount of dollars spent in the gallery (very important!), and the artists and groups of art that the customer tends to like.

Fine-Granular Design-Specific ERD

Questions Which part of this is least clear/most challenging? Which parts of the presented ERD did you do differently?

The last few weeks Presentation Layer ER Diagram Contains hashes and oval Design-specific ER Diagram (Coarse-granularity) Uses (min, max) notation Maps deletion rules Design-specific ER Diagram (Fine-granularity) Maps attribute characteristics into ER diagram Decomposes multi-valued attributes Decomposes m:n relationships

The missing piece… Enhanced Entity-Relationship (EER) extends the ERD by incorporating additional constructs, specifically the superclass/subclass (SC/sc) relationship

An example of a superclass/subclass relationships (SC/sc) Image from Data Modeling and Database Design, By Narayan Umanath, Richard Scamell

Why bother with superclasses/subclasses? This isn’t the only option for this example, e.g. we could have… Modelled furniture as one entity type with an attribute called furniture_type; then chair, table, and sofa would be values of that attribute Modelled three independent entity types for chair, table, and sofa, and create three separate relationship types with the entity type store The first options creates redundancy, the second struggles with the relationship constraints for large numbers of sub-classes

A vignette Image from Data Modeling and Database Design, By Narayan Umanath, Richard Scamell

Option 1: modelling student athletes with composite attributes Image from Data Modeling and Database Design, By Narayan Umanath, Richard Scamell

Option 2: modelling student athletes using inter-entity class relationships Image from Data Modeling and Database Design, By Narayan Umanath, Richard Scamell

Types of SC/sc relationships There are two basic kinds of Sc/sc relationships Specialization/Generalization: One superclass (SC) is related to one or more subclasses (sc) Categorization: One subclass (sc) is related to one or more superclasses (SC)

Specialisation and Generalisation Specialisation is the process of generating subgroups (‘sc’s) of a generic entity class (SC) by specifying the distinguishing properties (attributes) of the subgroups (= top-down approach) Generalisation, on the other hand, crystallizes the common properties (attributes) shared by a set of entity types (‘sc’s) into a generic entity type (SC) (= bottom-up approach) Notation: circle + fork (indicating subset) Read: “is-a”

Two important constraints of specialisation and generalisation Completeness Constraint can be one of two values Total specialisation means that every entity of the superclass must participate in this specialisation/ generalisation relationship (indicated by a solid line from the superclass to the specialization/generalization symbol (i.e., the circle)) Partial specialisation means that there may be entities present in the superclass that do not participate in this specialisation/generalisation (indicated by a dotted line)

Two important constraints of specialisation and generalisation Disjointness Constraint can also be one of two values Disjointed means an entity of the superclass cannot be a member of more than one subclass (indicated by ‘D’), Overlapped means an entity of the superclass can be a member of more than one subclass (indicated by ‘O’)

Notation for completeness and disjointness constraints Image from Data Modeling and Database Design, By Narayan Umanath, Richard Scamell We haven’t come to this yet

Notation for completeness and disjointness constraints Image from Data Modeling and Database Design, By Narayan Umanath, Richard Scamell

Notation for completeness and disjointness constraints Image from Data Modeling and Database Design, By Narayan Umanath, Richard Scamell

Categorisation and Aggregation Specialisation/ Generalisation d Specialisation Generalisation U U d Aggregation Categorisation U U

Categorisation and Aggregation The first form of this is a category construct Notation: ‘U’ Image from Data Modeling and Database Design, By Narayan Umanath, Richard Scamell

Categorisation and Aggregation The second form of this is an aggregation construct Notation: ‘A’ (note that an aggregate can never be partial) Image from Data Modeling and Database Design, By Narayan Umanath, Richard Scamell

An aggregate hierarchy Image from Data Modeling and Database Design, By Narayan Umanath, Richard Scamell

Example of Presentation Layer EERD Image from Data Modeling and Database Design, By Narayan Umanath, Richard Scamell

A Fine-granular Design-Specific EER Diagram for the vignette Image from Data Modeling and Database Design, By Narayan Umanath, Richard Scamell

Exercise For the Design-Specific EER Diagram for the vignette on the previous slide What are the superclass entity types and subclass entity types? What attributes are inherited by the entity type Pitcher

Exercise Draw a Presentation Layer ER Diagram for the following narrative A commercial air flight stores a flight list containing both passengers and staff. This flight list detail the final destination and passport number for everyone on board. Passengers will also have data for their name, nationality and seat number. Staff will also have data for their name, job title and the number of years they have worked for the company. Some staff are contract staff, in which case their pay rate and employment agency should be listed. Other employees are permanent staff, in which case their salary, pension details, and union should be listed. Other employees are non-paid government officials, in which case their department and clearance level should be listed. For paid staff (meaning contract and permanent staff), an employee number must be stored and home location may also be listed.