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Data Modelling

Which data to include in the database

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Aim of Data Modelling

Form a model of the enterprise so that the data are accurate and useful.

Yet another form of abstraction. First and most important step in database design. Requires a good understanding of enterprise being

modeled. Different methods of data modeling, but the most

widely used is the Entity-Relationship (E-R) model.

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Entity-Relationship Model

Primarily used to specify an external view of the database.

Recall that the conceptual schema can be created out of the various external views.

Model the world in terms of entities relationships between entities attributes of entities and relationships.

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Elements in the E-R Model I

Entity: A distinguishable thing, person or event

Entity set A set of entities of the same type

Relationship Representation of the fact that certain entities are

related to each other in a specific way. Relationship set

Set of relationships of a given type.

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Elements in the E-R Model II

Attributes: Properties of entities or relationships Attribute values are drawn from a given domain or

value set Attributes may have a set of values (multi-valued

or set-valued attributes). Key:

An attribute or set of attributes that uniquely identifies each entity in an entity set.

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E-R Diagrams

E-R models can be visualized in E-R diagrams. There are different conventions for drawing E-R

diagrams. The following is widely used: Rectangles for entity sets Ellipses for attributes Diamonds for relationship sets Lines to link

Each component is labelled with its corresponding name.

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Example of an E-R Diagram I UWI offers many courses. Each course has a unique course number and a

name. Students can enroll in many courses and a

course can be taken by many students. A student has a name and a unique student

number. Student obtain grades on courses. A course is taught by a single instructor.

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Example of an E-R Diagram II

Course

Instructor Student

Teaches Enrol

c num

c name

grade

i num

i name

s num

s_name

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Constraints I

In addition to the entity and relationship sets and their attributes, during data modelling, one also identifies constraints.

Examples: A course is taught by a single instructor. No student can enroll for more than 4 courses.

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Constraints II

Usually, constraints are not captured in the E-R diagram and need to be written down.

Two exceptions: Mapping cardinalities

Concerns the number of entities involved in a relationship

Existence dependencies Concerns the fact that sometimes the existence of an

entity depends on the existence of another entity.

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One-to-One Relationships

Each entity in E1 is related to at most one entity in E2 and vice versa.

Example: A department has only one head and no person

can be head of more than one department.

Department EmployeeHeadOf

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One-to-Many Relationships

An entity in E2 is related to at most one entity in E1, but an entity in E1 can be related to many entities in E2.

Example: A student has one professor as their supervisor but

a professor may supervise many students.

Student ProfessorSupervises

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Many-to-many Relationships

An entity can be related to many entities in the other set, and vice versa.

Example: A student can be enrolled in many courses, a

course can have many students enrolled in it.

Student CourseEnroll

grade

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Different Conventions

There are different conventions for mapping cardinalities. For example, instead of a 1 on the link, one may use an arrow pointing into the entity.

Department EmployeeHead

Of

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Existence Dependencies

An entity x is existence dependent on an entity y if the existence of x is dependent on the existence of y.

Y is called the dominant entity; x the subordinate entity.

Example: In a personnel database, a dependent (e.g., child) is existence dependent on the employee. If the employee is deleted, so is the dependent.

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Representation of Subordinate Entities Subordinate entities are usually represented in a

double box

Employee Dependent

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Some Complicationsto the E-R Diagram An entity set may stand in some relationship to

itself. The links are then labelled with the role that the

entity plays.

Person

Child-of

Parent Child

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Multiple Relationships Between Entity Sets

Course Student

Enroll

Tutor for

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Ternary Relationships

A customer may has an account at a particular branch.

Branch

Customer Account

CAB

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Generalizations

Inheritance hierarchies

Employee

AcademicStaff

SupportStaff

IS A

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Specialization

Sometimes, you want to make a lower level entity from a higher level entity. However, not every high-level entity must also be a lower-level entity.

Student

Grad Student

IS A

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Aggregation

Sometimes, you want to treat a relationship as an entity in its own right, e.g., to allow modelling of relationships between relationships.

Example: A customer is assigned a loan manager for every

loan that they get. Note that the relationship from the login is not with

the customer alone, or with the loan.

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Aggregation in an E-R Model

Customer Loanborrows

Employee

Loan-Manager

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Problems in Data Modelling

Should a real-world concept be modelled as an entity set or a relationship set?

Should a real-world concept be modelled as an attribute or an entity set?

Is the use of a generalization or aggregation appropriate?