1. Entity Relationship (ER) Modeling
Chapter 4
Entity Relationship (ER) Modeling
The ER Model
Developing and ER Diagram
Extended ER Model
Entity Integrity : Selecting Primary Key

2. In this chapter, you will learn:
The main characteristics of entity relationship components
How relationships between entities are defined and refined and how those relationships are incorporated into the database design process
How ERD components affect database design and implementation
That real-world database design often requires the reconciliation of conflicting goals
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Chapter 3 | Entity Relationship (ER) Modeling

3. The Entity Relationship (ER) Model
ER model forms the basis of an ER diagram
ERD represents conceptual database as viewed by end user
ERDs depict database’s main components:
Entities
Attributes
Relationships
Chapter 3 | Entity Relationship (ER) Modeling
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4. Entities Refers to entity set and not to single entity occurrence
Corresponds to table and not to row in relational environment
In both Chen and Crow’s Foot models, entity is represented by rectangle containing entity’s name
Entity name, a noun, is usually written in capital letters
Entities
Chapter 3 | Entity Relationship (ER) Modeling
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5. Attributes Characteristics of entities
In Chen model, attributes are represented by ovals and are connected to entity rectangle with a line
Each oval contains the name of attribute it represents
In Crow’s Foot model, attributes are written in attribute box below entity rectangle
Attributes
Chapter 3 | Entity Relationship (ER) Modeling
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6. Attributes (continued)
Chapter 3 | Entity Relationship (ER) Modeling
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7. Domains Attributes have domain
Domain is attribute’s set of possible values
Attributes may share a domain
a set of permitted values
- ex) domain of customer--name : set of all text strings
of a certain length
Domains are the set of allowable data values for a Column.
For example, the FiveDigitZipCode Column on the customer entity can be in the integer domain. As such, the database would not allow you to place values like (floating point) or ABC (character) into that Column.
Domains
Chapter 3 | Entity Relationship (ER) Modeling
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8. Identifiers (Primary Keys)
Underlined in the ERD
Key attributes are also underlined in frequently used table structure shorthand
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9. Primary Key Guidelines
Attribute or combination of attributes that uniquely identifies entity instances in an entity set
Main function is to uniquely identify an entity instance or row within a table
Guarantee entity integrity, not to “describe” the entity
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Chapter 3 | Entity Relationship (ER) Modeling

10. Primary Key Guidelines (continued)
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Chapter 3 | Entity Relationship (ER) Modeling

11. Composite Primary Keys
Primary keys ideally composed of only single attribute
Possible to use a composite key
Primary key composed of more than one attribute
Chapter 3 | Entity Relationship (ER) Modeling
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12. Composite Primary Keys (continued)
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13. When to Use Composite Primary Keys
Useful as identifiers of composite entities, where each primary key combination is allowed only once in M:N relationship
Automatically provides benefit of ensuring that there cannot be duplicate values
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Chapter 3 | Entity Relationship (ER) Modeling

14. When to Use Composite Primary Keys (continued)
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15. When to Use Composite Primary Keys (continued)
Useful as identifiers of weak entities, where weak entity has strong identifying relationship with parent entity
Normally used to represent:
A real-world object that is existent dependent on another real-world object
A real-world object that is represented in data model as two separate entities in strong identifying relationship
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Chapter 3 | Entity Relationship (ER) Modeling

16. Composite and Simple Attributes
Composite attribute can be subdivided
Simple attribute cannot be subdivided
Chapter 3 | Entity Relationship (ER) Modeling
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17. Single-Valued Attributes
Single-value attribute can have only a single value
Single-Valued Attributes
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18. Multivalued Attributes
Multivalued attributes can have many values
Multivalued Attributes
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19. Multivalued Attributes (continued)
Chapter 3 | Entity Relationship (ER) Modeling
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20. Resolving Multivalued Attribute Problems
Although conceptual model can handle M:N relationships and multivalued attributes, you should not implement them in relational DBMS
Within original entity, create several new attributes, one for each of the original multivalued attribute’s components
Can lead to major structural problems in table
Create new entity composed of original multivalued attribute’s components
Chapter 3 | Entity Relationship (ER) Modeling
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21. Resolving Multivalued Attribute Problems (continued)
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22. Resolving Multivalued Attribute Problems (continued)
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23. Resolving Multivalued Attribute Problems (continued)
Chapter 3 | Entity Relationship (ER) Modeling
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24. Derived Attributes
Attribute whose value may be calculated (derived) from other attributes
Need not be physically stored within database
Can be derived by using an algorithm
Chapter 3 | Entity Relationship (ER) Modeling
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25. Derived Attributes (continued)
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26. Derived Attributes (continued)
Chapter 3 | Entity Relationship (ER) Modeling
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27. Relationships Association between entities
Participants are entities that participate in a relationship
Relationships between entities always operate in both directions
Relationship can be classified as 1:M
Relationship classification is difficult to establish if know only one side of the relationship
Chapter 3 | Entity Relationship (ER) Modeling
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28. Connectivity and Cardinality
Used to describe the relationship classification
Cardinality
Expresses minimum and maximum number of entity occurrences associated with one occurrence of related entity
Established by very concise statements known as business rules
Chapter 3 | Entity Relationship (ER) Modeling
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29. Connectivity and Cardinality (continued)
Chapter 3 | Entity Relationship (ER) Modeling
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30. Existence Dependence Existence dependence Existence independence
Exist in database only when it is associated with another related entity occurrence
Existence independence
Entity can exist apart from one or more related entities
Sometimes refers to such an entity as strong or regular entity
Chapter 3 | Entity Relationship (ER) Modeling
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31. Relationship Strength
Weak (non-identifying) relationships
Exists if PK of related entity does not contain PK component of parent entity
Strong (Identifying) Relationships
Exists when PK of related entity contains PK component of parent entity
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Chapter 3 | Entity Relationship (ER) Modeling

32. Weak (Non-Identifying) Relationships
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33. Weak (Non-Identifying) Relationships (continued)
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34. Strong (Identifying) Relationships
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35. Weak Entities Weak entity meets two conditions
Existence-dependent
Cannot exist without entity with which it has a relationship
Has primary key that is partially or totally derived from parent entity in relationship
Database designer usually determines whether an entity can be described as weak based on business rules
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Chapter 3 | Entity Relationship (ER) Modeling

36. Weak Entities (continued)
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37. Weak Entities (continued)
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38. Relationship Participation
Optional participation
One entity occurrence does not require corresponding entity occurrence in particular relationship
Mandatory participation
One entity occurrence requires corresponding entity occurrence in particular relationship
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Chapter 3 | Entity Relationship (ER) Modeling

39. Relationship Participation (continued)
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40. Relationship Participation (continued)
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41. Relationship Participation (continued)
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42. Relationship Participation (continued)
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43. Relationship Degree
Indicates number of entities or participants associated with a relationship
Unary relationship
Association is maintained within single entity
Binary relationship
Two entities are associated
Ternary relationship
Three entities are associated
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44. Relationship Degree (continued)
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45. Relationship Degree (continued)
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46. Recursive Relationships
Relationship can exist between occurrences of the same entity set
Naturally found within unary relationship
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Chapter 3 | Entity Relationship (ER) Modeling

47. Recursive Relationships (continued)
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48. Recursive Relationships (continued)
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49. Recursive Relationships (continued)
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50. Recursive Relationships (continued)
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51. Recursive Relationships (continued)
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52. Recursive Relationships (continued)
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53. Composite Entities Also known as bridge entities
Composed of primary keys of each of the entities to be connected
May also contain additional attributes that play no role in connective process
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54. Composite Entities (continued)
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55. Composite Entities (continued)
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56. Composite Entities (continued)
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57. Developing an ER Diagram
Database design is iterative rather than linear or sequential process
Iterative process
Based on repetition of processes and procedures
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Chapter 3 | Entity Relationship (ER) Modeling

58. Developing an ER Diagram (continued)
Building an ERD usually involves the following activities:
Create detailed narrative of organization’s description of operations
Identify business rules based on description of operations
Identify main entities and relationships from business rules
Develop initial ERD
Identify attributes and primary keys that adequately describe entities
Revise and review ERD
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Chapter 3 | Entity Relationship (ER) Modeling

59. Developinag an ER Diagram (continued)
Tiny College
Tiny College is divided into several schools
Each school is composed of several departments
Each department may offer courses
Each department may have many professors assigned to it
Each professor may teach up to four classes; each class is section of course
Student may enroll in several classes, but (s)he takes each class only once during any given enrollment period
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Chapter 3 | Entity Relationship (ER) Modeling

60. Developing an ER Diagram (continued)
Tiny College (continued)
Each department has several students
Each student has only a single major and is associated with a single department
Each student has an advisor in his or her department
Each advisor counsels several students
The relationship between class is taught in a room and the room in the building
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61. Developing an ER Diagram (continued)
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62. Developing an ER Diagram (continued)
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63. Developing an ER Diagram (continued)
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64. Developing an ER Diagram (continued)
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65. Developing an ER Diagram (continued)
Chapter 3 | Entity Relationship (ER) Modeling
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66. Developing an ER Diagram (continued)
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67. Developing an ER Diagram (continued)
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68. Developing an ER Diagram (continued)
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69. Developing an ER Diagram (continued)
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70. Developing an ER Diagram (continued)
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71. Developing an ER Diagram (continued)
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72. Data Modeling Checklist
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Chapter 3 | Entity Relationship (ER) Modeling

73. Summary Entity relationship (ER) model
Uses ERD to represent conceptual database as viewed by end user
ERM’s main components:
Entities
Relationships
Attributes
Includes connectivity and cardinality notations
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Chapter 3 | Entity Relationship (ER) Modeling

74. Summary (continued)
Connectivities and cardinalities are based on business rules
In ERM, M:N relationship is valid at conceptual level
ERDs may be based on many different ERMs
Database designers are often forced to make design compromises
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Chapter 3 | Entity Relationship (ER) Modeling

75. Summary
Extended entity relationship (EER) model adds semantics to ER model via entity supertypes, subtypes, and clusters
Specialization hierarchy depicts arrangement and relationships between entity supertypes and entity subtypes
Inheritance allows an entity subtype to inherit attributes and relationships of supertype
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Chapter 3 | Entity Relationship (ER) Modeling

76. Summary (continued)
Composite keys are useful to represent M:N relationships and weak (strong-identifying) entities
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Chapter 3 | Entity Relationship (ER) Modeling

77. Summary (continued)
Surrogate primary keys are useful when there is no natural key that makes a suitable primary key
In a 1:1 relationship, place the PK of mandatory entity as foreign key in optional entity
Time-variant data refers to data whose values change over time and whose requirements mandate that you keep a history of data changes
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Chapter 3 | Entity Relationship (ER) Modeling

78. Summary (continued)
Data modeling checklist provides way for designer to check that the ERD meets set of minimum requirements
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Chapter 3 | Entity Relationship (ER) Modeling