1. Chapter 4 Entity Relationship (E-R) Modeling
Database Systems: Design, Implementation, and Management
Peter Rob & Carlos Coronel

2. In this chapter, you will learn:
How relationships between entities are defined and refined, and how such relationships are incorporated into the database design process
How ERD components affect database design and implementation
How to interpret the modeling symbols for the four most popular ER modeling tools
That real-world database design often requires that you reconcile conflicting goals

3. The Entity Relationship (E-R) Model
ER model forms the basis of an ER diagram
ERD represents the conceptual database as viewed by end user
Main Components
Entities
In E-R models an entity refers to the entity set.
An entity is represented by a rectangle containing the entity’s name.
Attributes
Attributes are represented by ovals and are connected to the entity with a line.
Each oval contains the name of the attribute it represents.
Attributes have a domain -- the attribute’s set of possible values.
Attributes may share a domain.
Relationships

4. The Attributes of the STUDENT Entity

5. Primary Keys Underlined in the ER diagram
Key attributes are also underlined in frequently used table structure shorthand
Ideally composed of only a single attribute
Possible to use a composite key:
Primary key composed of more than one attribute

6. The CLASS Table (Entity) Components and Contents

7. The Entity Relationship (E-R) Model
Classes of Attributes
A simple attribute cannot be subdivided.
Examples: Age, Sex, and Marital status
A composite attribute can be further subdivided to yield additional attributes.
Examples:
ADDRESS Street, City, State, Zip
PHONE NUMBER  Area code, Exchange number

8. The Entity Relationship (E-R) Model
Classes of Attributes
A single-valued attribute can have only a single value.
Examples:
A person can have only one social security number.
A manufactured part can have only one serial number.
Multivalued attributes can have many values.
A person may have several college degrees.
A household may have several phones with different numbers
Multivalued attributes are shown by a double line connecting to the entity.

9. Multivalued attributes in an Entity

10. The Entity Relationship (E-R) Model
Multivalued Attribute in Relational DBMS
The relational DBMS cannot implement multivalued attributes.
Possible courses of action for the designer
Within the original entity, create several new attributes, one for each of the original multivalued attribute’s components ( Figure 4.4).
Create a new entity composed of the original multivalued attribute’s components ( Figure 4.5).

11. Splitting the Multivalued Attribute into New Attributes

12. A New Entity Set Composed of Multivalued Attribute’s Components

13. The Entity Relationship (E-R) Model
A derived attribute
may be calculated (derived) from other attributes
Need not be physically stored within the database
Can be derived by using an algorithm
Example: AGE can be derived from the data of birth and the current date.

14. Depiction of a Derived Attribute

15. The Entity Relationship (E-R) Model
Relationships
A relationship is an association between entities.
Relationships are represented by diamond-shaped symbols.

16. The Entity Relationship (E-R) Model
Connectivity
The term connectivity is used to describe the relationship classification (e.g., one-to-one, one-to-many, and many-to-many).

17. The Entity Relationship (E-R) Model
Cardinality
Cardinality expresses the specific number of entity occurrences associated with one occurrence of the related entity.
The minimum and maximum number of entity occurrences
(0,3) for PROFESSOR : a professor may teach 0-3 classes.
(0,35) for CLASS : a class may enroll up to 35 students,
but it is possible for a class to have no currently enrolled students initially.

18. Connectivity and Cardinality in an ERD

19. Relationship Strength
Existence Dependent
If an entity’s existence depends on the existence of one or more other entities, it is said to be existence-dependent.
CLASS is existence-dependent on COURSE (parent entity)
EMPLOYEE claims DEPENDENT— DEPENDENT is existence-dependent on EMPLOYEE
Existence independent
Entity can exist apart from one or more related entities
Example:
some of parts are produced “in-house” and other parts are bought from vendors. At least some of the parts are not supplied by a vender.
PART is existence-independent from VENDOR

20. Relationship Strength
Weak (non-identifying) relationship
One entity is not existence-independent on another entity
PK of related entity doesn’t contain PK component of parent entity
COURSE( CRS_CODE, …)
CLASS( CLASS_CODE, …)
Strong (identifying) relationship
One entity is existence-dependent on another
PK of related entity contains PK component of parent entity
CLASS( CRS_CODE, CLASS_SECTION, …)

21. A Weak Relationship Between COURSE and CLASS

22. A Strong (Identifying) Relationship Between COURSE and CLASS

23. The Entity Relationship (E-R) Model
Relationship Participation
Optional
The participation is optional if one entity occurrence does not require a corresponding entity occurrence in a particular relationship.
An optional entity is shown by a small circle on the side of the optional entity.
Mandatory
Entity occurrence requires corresponding occurrence in related entity
If no optionality symbol is shown on ERD, it is mandatory

24. CLASS is Optional to COURSE
COURSE is Mandatory to CLASS
COURSE and CLASS in a Mandatory Relationship

25. Relationship Strength and Weak Entities
A weak entity is an entity that
Is existence-dependent and
Has a primary key that is partially or totally derived from the parent entity in the relationship.
The existence of a weak entity is indicated by a double rectangle.
The weak entity inherits all or part of its primary key from its strong counterpart.

26. A Weak Entity in an ERD
EMPLOYEE( EMP_NUM, EMP_LNAME, EMP_FNAME, EMP_INITIAL, EMP_DOB )
DEPENDENT( EMP_NUM, DEP_NUM, DEP_FNAME, DEP_DOB ) Primary Key
DEP_NUM

27. Weak entity in a Strong Relationship Between DEPENDENT and EMPLOYEE
( EMP_NUM )
( EMP_NUM + DEP_NUM )

28. Weak entity in a Strong Relationship
Weak relationship
One entity is not existence-independent on another
PK of related entity doesn’t contain PK component of parent entity
COURSE( CRS_CODE, …)
CLASS( CLASS_CODE, …)
Strong relationship
One entity is existence-dependent on another
PK of related entity contains PK component of parent entity
CLASS( CRS_CODE, CLASS_SECTION, …)
In any case, CLASS is always existence-dependent on COURSE, whether or not it is defined to be weak.
Not Weak entity

29. Relationship Degree
A relationship’s degree indicates the number of associated entities or participants.
A unary relationship exists when an association is maintained within a single entity.
A binary relationship exists when two entities are associated.
A ternary relationship exists when three entities are associated.

30. The Implementation of a Ternary Relationship
Researchers

31. The Entity Relationship (E-R) Model
Recursive relationship
A recursive relationship is one in which a relationship can exist between occurrences of the same entity set.
A recursive entity is found within a unary relationship.

32. 1:1 Recursive relationship
EMPLOYEE is married to EMPLOYEE

33. 1:M Recursive relationship PART contains PART
each part is used to create only one rotor assembly
C130 = 4× AA × AB …

34. M:N Recursive relationship
PART Contains PART
A part_ can be used to create several different kinds of other parts
A part_ is itself composed of many parts.
PART
PART
contains
PART
contains
PART
PART
COMPONENT

36. M:N Recursive relationship
COURSE Requires COURSE

37. Implementation of the 1:M “EMPLOYEE Manages EMPLOYEE” Recursive Relationship

38. The Entity Relationship (E-R) Model
Composite Entities
A composite entity is composed of the primary keys of each of the entities to be connected.
The composite entity serves as a bridge between the related entities.
The composite entity may contain additional attributes.

39. The Entity Relationship (E-R) Model
Composite Entities

40. Converting the M:N Relationship Into Two 1:M Relationships

41. At the start of registration
A class may exist even though it contains no students at all
A student has not yet signed up for any classes.

42. A Composite Entity in the ERD

43. The Entity Relationship (E-R) Model
Entity Supertypes and Subtypes
Describing the different types of employees within a single entity would be awkward at best.
Example : Aviation business ( Figure 4.27) the special pilot characteristics (EMP_LICENCE, EMP_RATING, EMP_MED_TYPE) would cause a large number of nulls for other employees who are not pilots.

44. Nulls Created by Unique Attributes

45. The Entity Relationship (E-R) Model
Generalization hierarchy
Depicts relationships between higher-level supertype and lower-level subtype entities.
Supertype contains the shared attributes
Subtype contains the unique attributes.
A subtype entity inherits its attributes and its relationships from the supertype entity.

46. A Generalization Hierarchy
Disjoint relationships are indicated by G

47. The Entity Relationship (E-R) Model
Disjoint Supertypes
Also known as non-overlapping subtypes
Subtypes that contain a subset of the supertype entity set
Each entity instance (row) of the supertype can appear in only one of the disjoint subtypes.
The supertype and its subtype(s) maintain a 1:1 relationship.

48. The EMPLOYEE/PILOT Supertype/Subtype Relationship

49. A Generalization Hierarchy with Overlapping Subtypes
Overlapping relationships are indicated by Gs

50. A Comparison of ER Modeling Symbols

51. SUMMARY

52. The Chen Representation of the Invoicing Problem

53. The Crow’s Foot Representation of the Invoicing Problem

54. The Rein85 Representation of the Invoicing Problem

55. The IDEF1X Representation of the Invoicing Problem

56. Developing an E-R Diagram
The process of database design is an iterative rather than a linear or sequential process.
Based on repetition of processes and procedures.
The basic E-R model is graphically depicted and presented for review.
The process is repeated until the end users and designers agree that the E-R diagram is a fair representation of the organization’s activities and functions.
Narrative- 敘述,講述

57. Developing an E-R Diagram
Tiny College Database (1)
Tiny College (TC) is divided into several schools. Each school is administered by a dean.
A 1:1 relationship exists between DEAN and SCHOOL.
Each dean is a member of a group of administrators (ADMINISTRATOR).
Deans also hold professorial rank and may teach a class ( PROFESSOR).
Administrators and professors are also Employees.

58. A Supertype/Subtype Relationship

59. Developing an E-R Diagram
Tiny College Database (0)
Most DBMS do not support supertype/subtype relationship directly.
At the implementation level, designers convert it into a 1:1 relationship.
A PROFESSOR is an EMPLOYEE.
An EMPLOYEE is not required to be a PROFESSOR.
PROFESSOR is optional to EMPLOYEE.
PROFESSOR is existence-dependent on EMPLOYEE, and it inherits its PK from EMPLOYEE. Therefore the relationship between EMPLOYEE and PROFESSOR is strong, while is PROFESSOR a weak entity.

60. A Supertype/Subtype Relationship in an ERD

61. Developing an E-R Diagram
Tiny College Database (1)
Each school is composed of several departments.
The smallest number of departments operated by a school is one,
and the largest number of departments is indeterminate (N).
Each department belongs to only a single school.

62. Developing an E-R Diagram
Tiny College Database (2)
Each department offers several courses.
courses is optional to department. (Some departments are research only.)

63. Developing an E-R Diagram
Tiny College Database (3)
A department may offer several classes of the same course.
A 1:M relationship exists between COURSE and CLASS.
CLASS is optional to COURSE

64. Developing an E-R Diagram
Tiny College Database (4)
Each department has many professors assigned to it.
One of those professors chairs the department. Only one of the professors can chair the department.
DEPARTMENT is optional to PROFESSOR in the “chairs” relationship.

65. Developing an E-R Diagram
Tiny College Database (5)
Each professor may teach up to four classes, each one a section of a course.
A professor may also be on a research contract and teach no classes.

66. Developing an E-R Diagram
Tiny College Database (6)
A student may enroll in several classes, but (s)he takes each class only once during any given enrollment period.
Each student may enroll in up to six classes and each class may have up to 35 students in it.
STUDENT is optional to CLASS.

67. Developing an E-R Diagram
Tiny College Database (8)
Each department has several students whose major is offered by that department.
Each student has only a single major and associated with a single department.
It is possible, at least for a while, for a student not to declare a major filed of study. DEPARTMENT is optional to STUDENT.

68. Developing an E-R Diagram
Tiny College Database (8)
Each student has an advisor in his or her department; each advisor counsels several students.
An advisor is also a professor, but not all professors advise students.

69. Components of the ER Model

70. Diagram from 6th edit.
Not yet updated.

71. The Completed Tiny College ERD

72. The Challenge of Database Design: Conflicting Goals
Database design must conform to design standards
High processing speeds are often a top priority in database design
Conflicting Goals
Design standards (design elegance)
Processing speed (high-transaction-speed)
require design compromises
Example: 1:1 supertype/subtype relationship
Two tables (avoid nulls)
a single table (high speed)
Operational requirements 經營上的

73. MAR_DATE

74. The Challenge of Database Design: Conflicting Goals
A recursive 1:1 relationship yields many different solutions.
Your job as a database designer is to use your professional judgment to yield a solution that meets the requirements.
Operational requirements 經營上的

75. Summary Entity relationship (ER) model
Uses ER diagrams to represent conceptual database as viewed by the end user
Three main components
Entities
Relationships
Attributes
Includes connectivity and cardinality notations
Connectivities and cardinalities are based on business rules

76. Summary (continued)
ER symbols are used to graphically depict the ER model’s components and relationships
ERDs may be based on many different ER models
Entities can also be classified as supertypes and subtypes within a generalization hierarchy
Database designers are often forced to make design compromises