Data Modeling

1Engineering and Technology Management

Data Modeling and ERD


Business rules

are derived from policies, procedures, events, functions, and other business objects, and state constraints on the organization.

govern how data are handled and stored.

govern creating, updating, and removing data in an information processing and storage system.

ABusiness rule is a statement that defines or constraints some aspects of the business.


Examples

“Every student in the university must have a faculty advisor.”

“A student may register for a section of a course only if s/he has successfully completed the prerequisities for that course.”

“A preferred customer qualifies for a 10 percent discount, unless he has an overdue account balance.”


Business rules

Assert business structure Control/influence business behavior Expressed in terms familiar to end users Automated through DBMS software


Declarative – what, not how Precise – clear, agreed-upon meaning Atomic – one statement Consistent – internally and externally Expressible – structured, natural language Distinct – non-redundant Business-oriented – understood by business

people

Business rules


First proposed by Chen 1976 Several times expanded Different notations available, most notably

Chen originalCrow foot

Entity-Relationship Model


Model is always a simplified depiction of reality Modeling always has a certain purpose

What might be important in one context might be uninteresting in another one

In creating a model, we are abstracting from the full complexity of real life We need to decide on which facts and relations to

keep, and which special cases to strip away

Models and Modeling


E-R model constructs

Entity Type – collection of entities that share common properties or characteristics (often corresponds to a table) Entity instance – a single occurence of an entity type

(often corresponds to a row in a table)

Attribute - property or characteristic of an entity type that is of interest to the organization (often corresponds to a field in a table)

Relationship type – link (association) between entity types (corresponds to primary key-foreign key equivalencies in related tables)

Relationship instance – association between entity instances


Sample E-R diagram


Entity symbols

Relationship symbols

Attribute symbols

A special entity that is also a relationship

Basic E-R notation


Entities

Person: EMPLOYEE, STUDENT, PATIENT

Place: STORE, WAREHOUSE, STATE

Object: MACHINE, BUILDING, AUTOMOBILE

Event: SALE, REGISTRATION, RENEWAL,

RETURN

Concept: ACCOUNT, COURSE, WORK CENTER


What should an entity be?

SHOULD BE: An object that will have many instances in the

database An object that will be composed of multiple

attributes An object that we are trying to model

SHOULD NOT BE: A user of the database system An output of the database system (e.g. a report)


Inappropriate entities

System userSystem user System outputSystem output

Appropriate entities


Entities

Exercise Identify possible entities in the following description. An Airline company wants to store and manage their

data on flights. They own several airplanes, each with a certain number of seats, and have a number of employees. Also they have customers in their frequent flyer program, each has a certain mileage in his/her account. The planes are assigned to certain flights, as well as employees, and customers can buy seats on the flights. Each flight has a destination, a time and date, and a number of miles.


Attributes

Attribute - property or characteristic of an entity type

Classifications of attributes: Simple versus Composite Attribute Single-Valued versus Multivalued Attribute Stored versus Derived Attributes Identifier Attributes


Identifiers (Keys)

Identifier (Key) - An attribute (or combination of attributes) that uniquely identifies individual instances of an entity type

Simple Key versus Composite Key Candidate Key – an attribute that could be a

key…satisfies the requirements for being a key


Characteristics of identifiers

Will not change in value Will not be null No intelligent identifiers (e.g. containing

locations or people that might change) Substitute new, simple keys for long,

composite keys


An attribute broken into component parts

A composite attribute


The key is underlined

Simple key attribute


The key is composed of two subparts

Composite key attribute


Entity with a multivalued attribute (Skill) and derived attribute (Years_Employed)

Derived from date employed and current date

Multivalued: an employee can have more than one skill


An attribute that is both multivalued and composite


Attributes

Exercise Identify possible attributes in the example from last

exercise. Find additional possible attributes. Decide on which attributes would be multi-valued, and which would be candidate keys.

An Airline company wants to store and manage their data on flights. They own several airplanes, each with a certain number of seats, and have a number of employees. Also they have customers in their frequent flyer program, each has a certain mileage in his/her account. The planes are assigned to certain flights, as well as employees, and customers can buy seats on the flights. Each flight has a destination, a time and date, and a number of miles.


Relationships

Relationship Types vs. Relationship Instances The relationship type is modeled as the diamond and

lines between entity types…the instance is between specific entity instances

Relationships can have attributes These describe features pertaining to the association

between the entities in the relationship Two entities can have more than one type of

relationship between them (multiple relationships)


Degree of relationships

Degree of a Relationship is the number of entity types that participate in it

Unary Relationship Binary Relationship Ternary Relationship


One entity related to another of the same entity type

Entities of two different types related to each other

Entities of three different types related to each other

Degree of relationships


Unary relationships


Binary relationships


Note: a relationship can have attributes of its own

Ternary relationships


Cardinality of relationships

One – to – One Each entity in the relationship will have exactly one

related entity One – to – Many

An entity on one side of the relationship can have many related entities, but an entity on the other side will have a maximum of one related entity

Many – to – Many Entities on both sides of the relationship can have

many related entities on the other side


Cardinality constraints

Cardinality Constraints - the number of instances of one entity that can or must be associated with each instance of another entity.

Minimum Cardinality If zero, then optional If one or more, then mandatory

Maximum Cardinality The maximum number


Cardinality


Basic relationship with only maximum cardinalities showing

Mandatory minimum cardinalities


Optional cardinalities with unary degree, one-to-one relationship


Relationship type

Entity and Relationship instances


A binary relationship with an attribute

Here, the date completed attribute pertains specifically to the employee’s completion of a course…it is an attribute of the relationship.


A ternary relationship with attributes


Representing a billofmaterials structure

A unary relationship with an attribute. This has a many-to-many relationship


Examples of multiple relationships – entities can be related to one another in more than one way

Employees and departments


Professors and courses (fixed upon constraint)

Here,max cardinality constraint is 4


Cardinalities

Exercise Find the right cardinalities...

Student

Id Name

Coursetakes

Professor

advises

Textbook

uses

Grade


Cardinalities

Exercise Infer the business rule difference(s) to Bogazici.

What does the first company not support compared to the second one?

CourseProfessor gives

ProductCustomer buys

DateDate Invoice

ProductCustomer buys

Invoice


Strong vs. Weak Entities, andidentifying relationships

Strong entities exist independently of other types of entities has its own unique identifier represented with single-line rectangle

Weak entity dependent on a strong entity…cannot exist on its own does not have a unique identifier represented with double-line rectangle

Identifying relationship links strong entities to weak entities represented with double line diamond


Strong and weak entities

Strong entity Weak entityIdentifying relationship


Supertype / Subtype and Subset

Supertype / Subtype Implements generalisation All attributes of Supertype pertain to Subtypes Normally split on one attribute

Subset Similar, but non-excluse One entity instance can belong to more than one class


Supertype/subtype relationships


ERD

Exercise Model the following situation using ERD. Include

appropriate attributes. A consulting company maintains offices in several

countries. Each office can have several employees assigned, each employee is assigned to one office. Customers offer projects to the company, each project is from one customer. Employees work on the project for a given time and intensity, and can work on more than one project at the same time. A project will have several employees assigned. Each employee has a list of skills. An employee can be either a senior or junior consultant (who has a date for promotion set), a senior one manages several junior one, each junior is assigned to one senior. For each project, there can be several invoices with different amounts and dates.

Documents

Data Modeling