1. Entity-Relationship Model
Chapter 2
Entity-Relationship Model
Chapter 11 & 12 in Textbook

2. Database Design
Steps in building a database for an application: 1. Understand real-world domain being captured. 2. Specify it using a database conceptual model (E/R,OO). 3. Translate specification to model of DBMS (relational). 4. Create schema using DBMS commands (DDL). 5. Load data (DML).
Real-world
domain
Conceptual
model
DBMS data
model
Create
Schema
(DDL)
Load data
(DML)
ER Model

3. Entity-Relationship Model (E/R)
A picture is worth a thousand words The Entity-Relationship model (ER) is a high-level description of the structure of the DB. The Entity-Relationship Diagram (ERD) is a graphical model for representing the conceptual model for the data. A E/R models the DB using three element types: - Entities - Attributes - Relationships
ER Model

4. ERD Example name number DOB dean of operate chairs advise teach offer
PROFESSOR
number
SCHOOL 1
DOB 1 1 1 1
(0,1)
(1,1)
(0,1)
dean of
(0,4)
(0,*) 1
(1,*)
operate
(1,1)
chairs
(0,*) M
advise
teach
DEPARTMENT 1
(1,1) 1
(1,*)
(1,*) 1
offer
(1,1)
(1,1) M M
(1,1) M N M M 1
enroll-in
CLASS
belong-to
COURSE
STUDENT
(0,*)
(0,*)
(1,1)
(0,*)
(1,1) M
code
hrs
has

5. Entities & Entity Type
Entity is an object that exists and is distinguishable from other objects.
e.g. person, company, course, university
Entity Type is a set of entities of the same type that share the same properties.
e.g. set of all persons, companies, trees, courses
STUDENT
COURSE
ER Model

6. Relationships & Relationship Types
A relationship associates 2 or more entities.
A relationship type is a set of associations between entity types.
STUDENT study COURSE
S r CO1
S r CO2
S r3
study
STUDENT
COURSE
ER Model

7. Degree of Relationship Type
Degree of relationship refers to number of participating entity types in a relationship.
A relationship of degree two (2 entity types) are binary.
A relationship of degree three (3 entity types) are ternary.
ER Model

8. Degree of Relationship Type
A relationship of degree two (2 entity types) are binary.
study
STUDENT
COURSE
ER Model

9. Degree of Relationship Type
A relationship of degree three (3 entity types) are ternary.
e.g. registration of a student in a course by a staff.
register
STUDENT
COURSE
STAFF
ER Model

10. Recursive Relationship
Recursive relationship is a relationship type where the same entity type participates more than once in a different role. It is a unary relationship.
COURSE
STUDENT
require
lead
ER Model

11. Roles
Role indicates the purpose that each participating entity type plays in a relationship. (e.g. prerequisite, requester)
COURSE
STUDENT
requester
prerequisite
leader
member
require
lead
ER Model

12. Roles
Role can be used when two entities are associated through more than one relationship to classify the purpose of each relationship.
manages
Manager
Branch office
BRANCH
STAFF
Branch office
Staff member
allocated
ER Model

13. Attributes
Attributes are descriptive properties for an entity type or a relationship type. All entities in one entity type have the same attributes.
name
DOB
St_no
Tel_no
STUDENT
ER Model

14. Attributes of Entities
name
DOB
name
hours
St_no
number
Tel_no
study
STUDENT
COURSE
ER Model

15. Attributes of Relationships
Start
End
Object
Employee
Company
contract
All relationships in one relationship type have the same attributes. Relationships can be distinguished not only by their attributes but also by their participating entities.
ER Model

16. Simple & Composite Attributes
Simple attribute is an attribute that have a single value with an independent existence. e.g. salary, age, gender,...
ER Model

17. Single-valued & Multi-valued Attributes
Multi-valued attribute is an attribute that may hold multiple values, of the same type, for a single entity. e.g. tel_no, degrees,…
initial
FName
LName
Area_cd
name
DOB no
St_no
Tel_no EX
STUDENT
ER Model

18. Simple & Composite Attributes
Composite attribute is an attribute composed of multiple distinct components, each with an independent existence.
e.g. address (street, area, city, post code)
name (First name, initial, Last name)
phone no. (area code, number, exchange no)
initial
FName
LName
Area_cd
name
DOB no
St_no
Tel_no EX
STUDENT
ER Model

19. Single-valued & Multi-valued Attributes
Single-valued attribute is an attribute that holds a single value for a single entity. It is not necessarily a simple attribute. e.g. student_no, age, gender,...
ER Model

20. Derived Attributes
Derived attribute is an attribute that represents a value that is derived from the value of a related attribute,not necessarily in the same entity type. e.g. age is derived from date_of_birth total_cost is derived from quantity*unit_price
name
DOB
St_no
Tel_no
age
STUDENT
ER Model

21. Keys
Candidate key (CK) is the minimal set of attributes that uniquely identifies an entity. It cannot contain null. e.g. student_no, social_security_no, branch_no… Primary Key (PK) is a candidate key that is selected to uniquely identify each entity. Alternate Key (AK) is a candidate key that is NOT selected to be the primary key.
ER Model

22. Keys Example
ELEMENT(symbol, name, atomic_no)
ER Model

23. Keys Example Candidate Key ELEMENT(symbol, name, atomic_no)
Primary Key
Alternate Keys
ER Model

24. Choice of PK Choice of Primary Key (PK) is based on: Attribute length
Number of attributes required
Certainty of uniqueness
ER Model

25. Primary Key in ERD STUDENT initial FName LName Area_cd name DOB no
St_no
Tel_no EX
STUDENT
age
ER Model

26. Keys A key can be: - simple key is a candidate key of one attribute.
e.g. student_no, branch_no…
- composite key is a candidate key that consists of two or more attributes.
e.g. STUDENT (Lname, Fname, Init)
CLASS (crs_code, section_no)
ADVERT (property_no, newspaperName, dateAdvert)
ER Model

27. Composite Key in ERD
name
Section_no
hours
crs_code
CLASS
ER Model

28. Strong & Weak Entity Types
A strong entity type is NOT existence-dependent on some other entity type. It has a PK. A weak entity type is an entity type that is existence-dependent on some other entity type. It does not have a PK.
ER Model

29. Weak Entity Type
The existence of a weak entity type depends on the existence of a strong entity set; it must relate to the strong entity type via a relationship type called identifying relationship.
The PK of a weak entity set is formed by the PK of its strong entity type, plus a weak entity type discriminator attribute.
LName
FName
dep_no
FName
emp_no
DOB
has
EMPLOYEE
DEPENDENT
ER Model

30. Cardinalities
Cardinality ratio expresses the number of relationships an entity can participate in. Most useful in describing binary relationship types. For a binary relationship type the mapping cardinality must be one of the following types: – One to one (1:1) – One to many(1:M) – Many to one (M:1) – Many to many (M:N)
ER Model

31. One-To-One Relationship
PROFESSOR chair DEPARTMENT
P1 r D002 P2
P3 r2 D001 1 1
chairs
PROFESSOR
DEPARTMENT
A professor chairs at most one department; and a department is chaired by only one professor.
ER Model

32. One-To-Many Relationship
PROFESSOR teach COURSE
P1 r C01
r2 C02
P2 r3 C03
P C04 1 M
teach
PROFESSOR
COURSE
A course is taught by at most one professor; a professor teaches many courses.
ER Model

33. Many-To-One Relationship
CLASS require ROOM
C r R001
C r R002
C r R003
R004 M 1
require
CLASS
ROOM
A class requires one room; while a room can be scheduled for many classes.
ER Model

34. Many-To-Many Relationship
CLASS enroll STUDENT
C r S1
C r S3
C r S4
r S5 M N
enroll
CLASS
STUDENT
A class enrolls many students; and each student is enrolled in many classes.
ER Model

35. Multiplicity
Multiplicity is the number (range) of possible entities that may relate to a single association through a particular relationship. It is best determined using sample data. Takes the form (min#, max#)
ER Model

36. Multiplicity STAFF manage BRANCH SG1 r1 B002 SG2 SG3 r2 B001 1 1
(0,1)
(1,1)
ER Model

37. Multiplicity STAFF oversee PROPERTY SG1 r1 P1 r2 P2 SG2 r3 P14 SG3 P6
(0,10)
(0,*)
(0,1)
ER Model

38. Multiplicity
Newspaper advertise PROPERTY Al-Riyadh r1 P1 r2 P13 Al-Bilad r3 Al-Madinah r4 P6 Al-Sharq P4 M N
advertise
NEWSPAPER
PROPERTY
(0,*)
(0,*)
ER Model

39. Participation Constraints
Participation constraints determine whether all or only some entities participate in a relationship. Two types of participation: - Mandatory (total) - Optional (partial)
ER Model

40. Participation Constraints
Mandatory (total) (1:*): if an entity’s existence requires the existence of an associated entity in a particular relationship (existence-dependent).
e.g. CLASS taught-by PROFESSOR
i.e. CLASS is a total participator in the relation.
A weak entity always has a mandatory participation constraints but the opposite not always true.
ER Model

41. Participation Constraints
Optional (partial) (0:*): if an entity’s existence does not require a corresponding entity in a particular relationship. (Not existence-dependent).
e.g. PROFESSOR teach CLASS
i.e. PROFESSOR is a partial participator in the relation.
ER Model

42. From Chen Notation to Crow’s Foot Notation
ER Model

43. Chen Notation
EntityName
Verb Phrase
AttributeName

44. Crow’s Foot Notation Acceptable Entity Attribute Relationship
EntityName
EntityName
Verb phrase
List of Attributes
Acceptable

45. ER diagram of Staff and Branch entities and their attributes
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46. Strong entity type called Client and weak entity type called Preference
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47. Relationship called Advertises with attributes
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48. Multiplicity of ternary Registers relationship
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49. Chen Model Crow’s Foot Cardinality 1 to represent one.
M to represent many
Crow’s Foot 1 M

50. Binary Relationships
The 1: M relationship between PAINTER and PAINTING

51. The 1:1 Relationship Between PROFESSOR and DEPARTMENT

52. The M:N Relationship Between STUDENT and CLASS

53. Mandatory vs. Optional Cardinalities
Specifies whether an instance must exist or can be absent in the relationship
Lecturer
Class
handles
A Lecturer may handle zero or many classes.
A class is handled by one and only one Lecturer.
Optional
Mandatory
(0,*)
(1,1) 1 M

54. Multiplicity as cardinality and participation constraints
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55. ERD Mistakes
ER Model

56. Problems with ER Models
Problems may arise when designing a conceptual data model called connection traps.
Often due to a misinterpretation of the meaning of certain relationships.
Two main types of connection traps are called fan traps and chasm traps.
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57. Problems with ER Models
Fan Trap
Where a model represents a relationship between entity types, but pathway between certain entity occurrences is ambiguous.
Chasm Trap
Where a model suggests the existence of a relationship between entity types, but pathway does not exist between certain entity occurrences.
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58. An Example of a Fan Trap
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59. Semantic Net of ER Model with Fan Trap
At which branch office does staff number SG37 work?
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60. Restructuring ER model to remove Fan Trap
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61. Semantic Net of Restructured ER Model with Fan Trap Removed
SG37 works at branch B003.
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62. An Example of a Chasm Trap
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63. Semantic Net of ER Model with Chasm Trap
At which branch office is property PA14 available?
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64. ER Model restructured to remove Chasm Trap
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65. Semantic Net of Restructured ER Model with Chasm Trap Removed
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