DATA MODELING USING THE ENTITY-RELATIONSHIP MODEL

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E-R model is a high-level conceptual model for database design

Example
1. COMPANY has departments
2. Department has name, number, manager
Manager is an employee
Manager has starting date
Department has several locations
3. Department controls projects
Project has name, number, location
4. Employee has name, social security, number, address, salary, sex, birthdate.
5. Employee is in one department but may work on several projects
6. Hours each employee works on each project
7. Employee has supervisor
8. Employee has dependents with name, sex, birthdate, and relationship to employee.

Phases of database design (simplified):

ENTITIES AND ATTRIBUTES

An entity is an object in the real world.

Each entity has properties, called attributes.

A particular entity has values for the attributes.

Attributes may be:
simple (atomic) (ZIP)
composite (ADDRESS)
single-valued (AGE)
multivalued (DEGREES)

Derived attribute:
AGE from BIRTHDATE

Attributes may have NULL values
unknown
not applicable

ENTITY TYPES

An entity type is the set of entities having the same attributes

We can describe an entity type by an entity type schema

The real entities in the entity type are entity instances
entity type <-> entity instances
object type <-> object instances
relation <-> tuples (relation instances)
intension <-> extension

Two entity types and some of the member entities:

KEY ATTRIBUTE OF AN ENTITY TYPE

A key attribute uniquely identifies an entity
The entity type is
(v1, v2)
Suppose the entities are:
(1, A)
(4, A)
(5, B)
The key attribute is the first attribute v1.

DOMAIN OF AN ATTRIBUTE
The domain of v1 is {1,4,5}
The domain of v2 is {A,B}
CARTESIAN PRODUCT
{1,4,5} X {A,B}
{1,A},{1,B},{4,A},{4,B},{5,A},{5,B}

The CAR entity type. Multivalued attributes are shown between braces {}. Components of a composite attribute are shown between ():

INITIAL CONCEPTUAL DESIGN OF THE COMPANY DATABASE

First we identify the entity types
DEPARTMENT
PROJECT
EMPLOYEE
DEPENDENT

Then we identify the attributes for each entity type

Multivalued attributes, composite attributes, derived attributes, attributes with possible nulls

Key attributes are identified or assumed.
For example, we assume each project has a unique project_number

Question: Why do we need key attributes?

Are there real life objects without key attributes?

RELATIONS, ROLES AND STRUCTURAL CONSTRAINTS

An entity type PARTICIPATES in a relationship type
eg. EMPLOYEE and DEPARTMENT participate in the relationship WORKS_FOR

DEGREE: the number of participating entity types
eg. WORKS_FOR has degree 2

ROLE_NAME: the role each participating entity plays in a relationship.
eg. EMPLOYEE plays the role "employee" in WORKS_FOR relationship

RECURSIVE ROLES: EMPLOYEE can play the role of both "employee" and "supervisor"

CONSTRAINTS: We use CARDINALITY RATIO to express a constraint on a relationship type, such as
1:1
1:N
M:N

PARTICIPATION CONSTRAINTS: partial or total PARTICIPATION CONSTRAINT
An employee MUST work for a department
An employee entity can exist only if it participates in a WORKS_FOR relationship instance
Thus its participation is TOTAL

Only some employees manage departments
The participation is PARTIAL

A formal constraint: (m, n) where
m, n are number of times

PARTICIPATION CONSTRAINT

An employee MUST work for a department
An employee entity can exist only if it participates in a WORKS_FOR relationship instance
Thus its participation is TOTAL

Only some employees manage departments
The participation is PARTIAL

A formal constraint: (min,max) where m, n are min and max number of times an entity participates in a relationship instance. For example, (0,10) means partial participation, and (1,max) means total participation.

Some instances of the WORK_FOR relationship:

The ternary relationship SUPPLY:

The recursive relationship SUPERVISION - EMPLOYEE plays two roles of "supervisor" and "supervisee":

The 1:1 relationship MANAGES, with partical participation of EMPLOYEE and total participation of DEPARTMENT:

The M:N relationship WORKS_ON:

ATTRIBUTES OF RELATIONSHIP TYPES

An employee entity WORKS_ON a project
The relationship is M:N
If we want to keep track of the number of hours an employee works for a project, it is difficult to keep it as an attribute of the employee entity
We can keep HOURS as an attribute of the WORKS_ON relationship type

An employee WORKS_FOR a department
The relationship is N:1
If we want to keep track of the starting date of an employee working for a department, where do we keep it?
IN SCHEMA DESIGN, THE DESIGNER MUST DECIDE WHERE TO KEEP ATTRIBUTES.

WEAK ENTITY TYPES

John Doe is an employee. He has a dependent Cindy Doe.
Mary Doe is an employee. She has a dependent Cindy Doe.
The two DEPENDENT entities are IDENTICAL!
Such entity types are called WEAK ENTITY TYPES, where entities may not be distinct.
(They still correspond to same real-life object)
Weak entity type must be OWNED by some owner entity type. For example, the EMPYLOEE entity type owns the DEPENDENT entity type.
How can you eliminate weak entity types?

INITIAL DESIGN OF COMPANY DATABASE

DEPARTMENT
Name, Number, {Locations}, Manager, ManagerStartDate

PROJECT
Name, Number, Location, ControllingDepartment

EMPLOYEE
Name, SSN, Sex, Address, Salary, BirthData, Department, Supervisor, {WorksOn(Project, Hours)}

DEPENDENT
Employee, DependentName, Sex, BirthDate, Relationship

Note: {} indicates multivalued attributes, and () indicates component attributes of a composite attribute

ADD RELATIONSHIP TYPES TO DATABASE SCHEMA

MANAGES (1:1)
EMPLOYEE partial
DEPARTMENT total
Attribute: StartDate

WORKS_FOR (1:N)
DEPARTMENT total
EMPLOYEE total

CONTROLS (1:N)
PROJECT total
DEPARTMENT partial

SUPERVISION (1:N) EMPLOYEE partial
EMPLOYEE partial

WORKS_ON (M:N)
EMPLOYEE total
PROJECT total
Attribute: Hours

DEPENDENTS_OF (1:N)
EMPLOYEE partial
DEPENDENT total
(Weak Entity Type)

The ER Diagram

ER diagram is a graphical design tool

ER schema diagram for the COMPANY database:

ER diaram for the COMPANY schema with all role names included and with structural constraints on relationships specified using the (min,max) notation:

Summary of ER diagram notation:

An example of ER diagram for the library is shown below, where a key icon indicates the relationship is linked to the key attribute, and a ring icon indicates the relationship is linked to an attribute (which could be part of the key).

A TERNARY RELATIONSHIP TYPE IS NOT EQUIVALENT TO THREE BINARY RELATIONS

CAN_TEACH: relates a course to the instructors who can teach that course
(i,c)

TAUGHT_DURING: relates a semester to the instructors who taught some course during that semester
(i,s)

OFFERED_DURING: relates a semester to the courses offered during that semester by ANY instructor
(s,c)

OFFERS: relates an instrcutor who offers a course during a semester
(i,c,s)
Constraint: (i,c,s) cannot exist unless (i,c),(i,s),(s,c) exist, but converse is not true.

The following figures illustrate (a) ternary relationship type SUPPLY; (b) three binary relationship types are not equivalent to one ternary relationship type SUPPLY; (c) another example of ternary versus binary relationship type:

A weak entitye type INTERVIEW with a ternary identifying relationship type:

THE UML MODEL

The Unified Modeling Language(UML) is an object-oriented analysis and design that evolved as a result of the combined work of James Rumbaugh, Grady Booch, and Ivar Jacobson. In 1997, a UML consortium(which include Microsoft, Hewlett-Packard, Oracle, MCI Systemhouse, IBM, IntelliCorp and more) submitted the 1.1 version of the UML to the Object Management Group(OMG). Since then, UML has become a de facto standard in the software industry, and it continues to evolve.

For database modeling, the most useful part of UML is the Class Diagram and Object Diagram. We can use these diagrams for object-oriented database design.

An example of the conceptual design of a library information system

A library is planning to develop a library information system. This library will serve only registered students. Each book has title, author, publisher, date-of-publication, ISBN number and a set of keywords. Each student has SSN, first name, last name, address, telephone number. A student can borrow a book for up to two weeks, so it is necessary to keep track of the date-of-borrow. He can then renew the book for another two weeks. Two renewals are allowed. After two renews, the student must return a borrowed book. If a book is overdue, the student must pay a fine of $0.5 a day.

ER Model for library information system

Entity type BOOK(ISBN,title,author,publisher,date-of-pub,{keywords}) Notice {keywords} is a multi-valued attribute. The key is ISBN.

Entity type STUDENT(SSN,fname,lname,address,telephone) The key is SSN.

Relation type BORROW(SSN,ISBN,date-of-borrow,no-of-renewal,fine)

The BORROW relation type is one-to-many. Participation of BOOK is partial Participation of STUDENT is partial

The UML Class Diagram

                        ---------------
                        |  Borrow     |
      ------------------|-------------|-------------------------
   1  |             N   |  SSN, ISBN  |   1               1    |
--------------          | data-borrow |                  ---------------
|   Student  |          | no-renewal  |                  |    Book     |
|------------|          |  telephone  |                  |-------------|
| SSN        |          |     fine    |                  |    ISBN     |
| fanme      |          |-------------|                  |   author    |
| lname      |          | borrow()    |                  |    title    |
| address    |          | renewal()   |                  | publisher   |
| telephone  |          | cal-fine9)  |                  | date-pub    |
|------------|          ---------------                  |  keywords   |
| add()      |                                           |-------------|
| delete()   |                                           |    add()    |
--------------                                           |  delete()   |
                                                         ---------------

The UML Object Diagram

                  -------------------
    --------------|Doe,book1: Borrow|-------------------------------
    |             -------------------                              |
    |                 -------------------                   -------------
    |    -------------|Doe,book2: Borrow|-----------        |book1: Book|
    |    |            -------------------          |        -------------
--------------                                     |
|Doe: Student|                              -------------
--------------                              |book2: Book|
                                            -------------