1. Modelling Data in Organisation Entity-Relationship Model
MGIS 641
Koç University

2. Learning objectives
Define the terms business rule, entity type, attribute, relationship, cardinality,strong/weak entity, gerund ...
Entity types, classification of attributes
Distinguish between unary, binary & ternary relationships
Model multi-valued attributes & repeating groups in an E-RD
Convert many to many relationships to associative entities
Model simple time-dependent data using time stamps in E-RD

3. Introduction
E-R model remains the main stream approach for conceptual data modeling
relative ease of use
CASE tool support
Entities-relationships natural modeling way in the real world
E-R model used as a tool for communication between end-users & DB designers during analysis phase
used to construct a conceptual data model - representation of the structure of DB independent of DBMS S/W.
Introduced by Chen no standard notation - evolving

4. DB development process
Planning
Enterprise data model
Analysis
Conceptual data model
Logical DB design
Logical data model
Physical DB design
Technology model
Implementation
DB & repositories

5. Business Rules
Statements that define or constrain some aspect of the business
A Good Business Rule is:
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

6. Introduction to E-R model
E-R model : A detailed, logical representation of the entities, associations & data elements for an organisation/business area
An E-R model is normally expressed as an E-R diagram which is a graphical representation of an E-R model
E-R model may also be used to represent enterprise data model a well as conceptual data model (detailed E-RD)
Major constructs of E-R D are entities, relationships & associated attributes

7. E-R Model Constructs
Entity instance - person, place, object, event, concept (often corresponds to a row in a table)
Entity Type – collection of entities (often corresponds to a table)
Attribute - property or characteristic of an entity type (often corresponds to a field in a table)
Relationship– link between entities 2

8. Sample E-R Diagram 3

9. E-R notation Basic symbols : Entity Relationship Primary key Attribute
Multivalued
attribute
Gerund
Relationship degree :
Ternary
Unary
Binary
Mandatory 1 cardinality
Many(M) cardinality
(1,2,.. ,many)
Optional 0 or 1
cardinality
Relationship cardinality:
Class-subclass relationship
Exclusive
relationship
Optional zero-many
cardinality (0,1,2, ..., many)

10. A special entity that is also a relationship
Basic E-R Notation
A special entity that is also a relationship
Entity symbols
Attribute symbols
Relationship symbols 8

11. Entities
Entity type (entity class): A collection of entities that share common properties or characteristics
e.g. EMPLOYEE, STUDENT
STATE, REGION, COUNTRY
MACHINE, BUILDING
SALE, REGISTRATION
ACCOUNT, COURSE
EMPLOYEE
COURSE
ACCOUNT
Entity Instance : A single occurrence of an entity type
Entity Type : EMPLOYEE
Attributes :EMPLOYEE NUMBER
NAME
ADDRESS
CITY
STATE
ZIP
Instance of EMPLOYEE
Ahmet Aslan
RumeliFeneri Yolu. No.5
Sarıyer
Istanbul
80860

12. 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)

13. Inappropriate entities
System output
System user
Appropriate entities

14. Attributes
Attribute : A named property or characteristic of an entity that is of interest to the organisation
e.g. STUDENT : STUDENTNO., NAME, ADDRESS, PHONE NO.
AUTOMOBILE : VEHICLE ID, COLOUR, WEIGHT
EMPLOYEE : EMPLOYEE NO. , NAME, ADDRESS, SKILL
Classifications of attributes:
Simple versus Composite Attribute
Single-Valued versus Multi-valued Attribute
Stored versus Derived Attributes
Identifier Attributes
NAME
SKILL
COLOUR

15. 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 6

16. 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 7

17. Candidate keys identifier selection
Every entity type must have an attribute/set of attributes that uniquely identifies each instance & clearly distinguishes that instance from other instances of the same type
Candidate key : an attribute/combination of attributes that uniquely identifies each instance of an entity type
e.g. STUDENT NO
Entity Type : GAME Entity Type : EMPLOYEE
Attributes Then EMPLOYEE No,
HOME TEAM NAME+ADDRESS => Candidate keys
VISITING TEAM
REFEREE
DATE
RESULT then HOME TEAM + VISTING TEAM =.> Candidate key
Identifier-Primary Key : A candidate key selected as the identifier for an entity - may not be null

18. Primary key selection
Choose a candidate key which will not change its value over the life of each instance of the entity type
e.g. NAME+ADDRESS could change
Choose a candidate key such that for each instance of the entity, the attribute/combinations of attributes is guaranteed to have valid values & not to be null
e.g. GAME type DATE may be used but fixture may not be ready
Avoid the use of intelligent keys, whose structure indicates classifications, locations ...
e.g. first two digits may indicate warehouse location, but they may change as the conditions change => invalid primary key
Consider substituting single-attribute keys for large composite keys
e.g. HOME TEAM+VISITING TEAM => GAME NO
NAME
ADDRESS
STUDENT NO
PHONE NO
STUDENT

19. An attribute broken into component parts
A composite attribute
An attribute broken into component parts 12

20. Simple key attribute
The key is underlined 14

21. Composite key attribute
The key is composed
of two subparts 15

22. Entity with a multi-valued attribute (Skill) and derived attribute (Years_Employed)
from date employed and current date
Multivalued:
an employee can have
more than one skill 13

23. an attribute that is both multi-valued and composite
This is an example of time-stamping 37

24. Multi-valued attributes
Multi-valued attribute : An attribute that can have more than one value for each entity instance
During conceptual design highlight them
Subsequently normalise entity data - remove multi-valued attributes & place them in a separate entity type
EMPLOYEE NO
NAME
ADDRESS
SKILL
EMPLOYEE

25. Relationships
Relationship : An association between the instances of one or more entity types that is of interest to the organisation
glue that holds the various E-R model components together
e.g. Registrar interested in tracking who has completed what courses => relationship completes
STUDENT
Completes
COURSE
Many to many

26. Relationships STUDENT NO. COURSE TITLE DATE COMPLETED 4545-5656
MGIS 688
JUNE 04
MGIS 408
SEPT 05
MGIS 698
OCT 04
Date Completed is not an attribute of STUDENT entity type since Student with
No has completed courses on different dates. It is not an attribute of
COURSE entity type since MGIS 641 may be completed on different dates.
It is a property of Completes relationship.
DATE COMPLETED
STUDENT
COURSE
Completes

27. Degree of relationship
Degree of a relationship : Number of entities that participate in that relationship
three common type of relationships
Unary
Binary
Ternary
e.g. Completes relationship between STUDENT & COURSE is a degree two relationship as it involves two entity types.

28. Unary relationship
Unary (recursive) relationship : relationship between the instances of one entity type
PERSON is
married to
One-to-One
EMPLOYEE
Manages
Has
ITEM
Components
One-to-many
Many-to-many
QUANTITY A V X Y U
(1)
(2)
(3) B Z W

29. Binary relationship
Binary relationship : Relationship between the instances of two entity classes.
Most common type of relationship encountered Is
EMPLOYEE
PARKING PLACE
Assigned
One-to-One
PRODUCT LINE
Contains
PRODUCT
One-to-many
STUDENT
Registers
COURSE
for
Many-to-many

30. Ternary relationship
Ternary relationship : Simultaneous relationship among the entities of three entity types
QUANTITY
WAREHOUSE
VENDOR
Ships
PART

31. Cardinalities in relationships
Cardinality : The number of instances of an entity type that can (must) be associated with each instance of any other entity type
MOVIE
MOVIE COPY
is stocked as
One-to-many

32. Minimum & maximum cardinalities
Minimum cardinality : Minimum no. of instances of an entity type that may be associated with each instance of any other entity type
Maximum cardinality : Maximum no. of instances of an entity type that may be associated with each instance of any other entity type
MOVIE
MOVIE COPY
is stocked as
One-to-many
max = one
max = many
minimum =0
minimum = one
(mandatory)
(optional)

33. examples of cardinalities
PATIENT
Has
PATIENT HISTORY
Mandatory cardinalities 3
EMPLOYEE
Is Assigned to
PROJECT
One optional one mandatory Is
PERSON
married to
Optional cardinalities

34. Associative Entity-Gerund
Many-to-many relationships may have attributes => they may be entities in disguise
Gerund (associative entity) : A many-to-many relationship that a data modeler chooses to model as an entity type with several associated one-to-many relationships with other entity types
SHIPMENT NO
QUANTITY
WAREHOUSE
VENDOR
SHIPMENT
PART

35. Associative Entities-Gerunds
It’s an entity – it has attributes
AND it’s a relationship – it links entities together

36. An associative entity (CERTIFICATE)
Associative entity involves a rectangle with a diamond inside.
Note that the many-to-many cardinality symbols face toward the associative entity and not toward the other entities 21

37. an associative entity – bill of materials structure
This could just be a relationship with attributes…it’s a judgment call 27

38. Ternary relationship as an associative entity36

39. Existence dependency
Existence dependency : An instance of an entity type cannot exist without the existence of an instance some other (related) entity
Existence dependency - result of mandatory one cardinality
Weak entity : An entity type that has an existence dependency
MOVIE
Is Stocked as
MOVIE COPY

40. Identifying relationship
Identifying relationship : A relationship in which the primary key of the parent entity is used as part of the primary key of dependent (child) entity
MOVIE NO
MOVIE NAME
MOVIE NO
COPY NO
MOVIE
Is Stocked as
MOVIE COPY
Two benefits of an identifying relationship :
1) Data integrity. Existence dependencies are enforced since the primary keys
is shared - a weak entity cannot exist unless the parent entity exists
2) Ease of access of dependent entity. e.g. if Movie No + Copy No known
movie copy can be located.

41. Strong and weak entities
Strong entity
Weak entity
Identifying relationship

42. Multiple relationships
entities can be related to one another in more than one way
Employees and departments 40

43. Another example multiple relationships -professors and courses
Here,max cardinality constraint is 4 41

44. Modeling multi-valued attributes
During later stages of conceptual design multi-valued attributes are removed from the entities - converted to a separate entity
EMPLOYEE NO
NAME
ADDRESS
SKILL
EMPLOYEE
SKILL NAME
ADDRESS
EMPLOYEE NO
NAME
SKILL
EMPLOYEE
Has

45. Repeating groups
Repeating Group : A set of two or more multivalued attributes that are logically related.
PATIENT CHART
Date of Visit Physician Symptom
1/5/05 Orkun Cold
2/5/05 Utku Fever
3/5/05 Arzu Soar Throat
Patient No :
Patient Name : OAY
Address :K.U. Sarıyer
PATIENT NO
NAME
ADDRESS
PATIENT
DATE OF VISIT
SYMPTOM
PHYSICIAN

46. Repeating Group Removed
NAME
PATIENT NO
ADDRESS
PATIENT NO
Has
PATIENT
PATIENT HISTORY
PHYSICIAN
SYMPTOM
DATE OF VISIT

47. Modeling Time Dependent Data
Time Stamp : A time value that is associated with any data value
PRODUCT NO
PRICE
EFFECTIVE DATE
PRODUCT
PRICE
EFFECTIVE DATE
PRODUCT NO
PRODUCT
Has
PRICE HISTORY
PRODUCT NO

48. Detailed E-R diagram for Pine Valley Furniture