1. Entity-Relationship Modeling
Chapter 12
Entity-Relationship Modeling

2. Chapter 12 - Objectives
How to use Entity–Relationship (ER) modeling in database design.
Basic concepts associated with ER model.
Diagrammatic technique for displaying ER model using Unified Modeling Language (UML).
How to build an ER model from a requirements specification.

3. ER diagram of Branch user views of DreamHome

4. Concepts of the ER Model
Entity types (Rectangles)
Relationship types (Links)
Attributes (Names inside rectangles)

5. Entity Type Entity type (Class)
Group of objects with same properties, identified by enterprise as having an independent existence.
Entity occurrence (Instance of a Class; Object)
Uniquely identifiable object of an entity type.

6. Examples of Entity Types (Nouns; appear in middle of box if alone, at top of box if attributes are listed)

7. ER diagram of Staff and Branch entity types

8. Relationship Types Relationship type Relationship occurrence
Meaningful associations among entity types.
i.e., Link between classes
Name occurs as label on the link
Arrow indicates direction of relationship type
Relationship occurrence
Uniquely identifiable association, which includes one occurrence from each participating entity type.
i.e., Link between instances

9. ER diagram of Branch Has Staff relationship type

10. Semantic net of Has relationship occurrence

11. Relationship Types Degree of a Relationship Relationship of degree :
Number of participating entities in relationship.
Relationship of degree :
two is binary
three is ternary
four is quaternary.

12. Binary relationship called POwns

13. Ternary relationship called Registers (add diamond instead of label)

14. Quaternary relationship called Arranges (add diamond instead of label)

15. Relationship Types Recursive Relationship
Relationship type where same entity type participates more than once in different roles.
Relationships may be given role names to indicate purpose that each participating entity type plays in a relationship.
End up with 2 labels on arrow (one per role)

16. Recursive relationship called Supervises with role names

17. Entities associated through two distinct relationships with role names

18. Attributes Attribute Attribute Domain
Property of an entity or a relationship type.
Line below relationship type name, separating name from attributes
Attribute Domain
Set of allowable values for one or more attributes.

19. Attributes Simple Attribute Composite Attribute
Attribute composed of a single component with an independent existence.
Composite Attribute
Attribute composed of multiple components, each with an independent existence.
Indent components under attribute name

20. Attributes Single-valued Attribute Multi-valued Attribute
Attribute that holds a single value for each occurrence of an entity type.
Add name under relationship type name
Multi-valued Attribute
Attribute that holds multiple values for each occurrence of an entity type.
Add number n or range [n..m] after attribute name

21. Attributes Derived Attribute Composite Attribute
Attribute that represents a value that is derivable from value of a related attribute, or set of attributes, not necessarily in the same entity type.
Add ‘/’ before attribute name
E.g., birthdate
/age
Composite Attribute
Indent the components

22. Keys
Candidate Key (add {CK} or {AK} for alternate key after attribute name)
Minimal set of attributes that uniquely identifies each occurrence of an entity type.
Primary Key (add {PK} after attribute)
Candidate key selected to uniquely identify each occurrence of an entity type.
Composite Key (unknowable; put {PK} after both attributes)
A candidate key that consists of two or more attributes.

23. ER diagram of Staff and Branch entities and their attributes

24. Relationship called Advertises with attributes (Note: No name on the box for relationship attributes)

25. Structural Constraints
Main type of constraint on relationships is called multiplicity.
Multiplicity - number (or range) of possible occurrences of an entity type that may relate to a single occurrence of an associated entity type through a particular relationship.
Represents policies (called business rules) established by user or company.

26. Structural Constraints
The most common degree for relationships is binary.
Binary relationships are generally referred to as being:
one-to-one (1:1)
one-to-many (1:*)
many-to-many (*:*)

27. Semantic net of Staff Manages Branch relationship type

28. Multiplicity of Staff Manages Branch (1:1) relationship

29. Semantic net of Staff Oversees PropertyForRent relationship type

30. Multiplicity of Staff Oversees PropertyForRent (0:*) relationship type

31. Semantic net of Newspaper Advertises PropertyForRent relationship type

32. Multiplicity of Newspaper Advertises PropertyForRent (. :
Multiplicity of Newspaper Advertises PropertyForRent (*:*) relationship

33. Structural Constraints
Multiplicity for Complex Relationships
Number (or range) of possible occurrences of an entity type in an n-ary relationship when other (n-1) values are fixed.

34. Summary of multiplicity constraints

35. Structural Constraints
Multiplicity is made up of two types of restrictions on relationships: cardinality and participation.

36. Structural Constraints
Cardinality
Describes maximum number of possible relationship occurrences for an entity participating in a given relationship type.
Participation
Determines whether all or only some entity occurrences participate in a relationship.

37. 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.

38. ER Modeling Process Get requirements from user
Create ER diagram (Conceptual modeling)
Nouns -> entities or attributes
Verbs -> relationships
Map from ER diagram to SQL tables (Logical modeling)
Entities -> tables
Attributes -> tables or attributes
Relationship attributes -> tables or attributes

39. Example
Draw the ER Diagram for the following online registration system:
The university offers courses uniquely identified by department and course number.
There may be multiple sections of a given course in a particular semester, including no offerings at all.
Sections are taught by professors in rooms (identified by building number and room number).
Rooms have specific capacities, but sections may have lower capacity (unused seats).
Students enroll in sections.

40. Example – Identify nouns
Draw the ER Diagram for the following online registration system:
The university offers courses uniquely identified by department and course number.
There may be multiple sections of a given course in a particular semester, including no offerings at all.
Sections are taught by professors in rooms (identified by building and room number).
Rooms have specific capacities, but sections may have lower capacity (unused seats).
Students enroll in sections.

41. Example1 – Select entities (in red) (Rest are attributes or discarded)
Draw the ER Diagram for the following online registration system:
The university offers courses uniquely identified by department and course number.
There may be multiple sections of a given course in a particular semester, including no offerings at all.
Sections are taught by professors in rooms (identified by building and room number).
Rooms have specific capacities, but sections may have lower capacity (unused seats).
Students enroll in sections.

42. ER Model
Course: dept. name, course number, title, credit hours, description, …
Section: sectionId, semester, professor name, Student: studentId, name, major, …
EnrolledIn: relationship between Student-Section
Offered: relationship between Section-Course
Relationship attributes: building name, room number, room capacity

43. Add Multiplicity
Student – Section
* : *
Course – Section
1 : *

44. Map to Tables
Course: dept. name, course number, title, credit hours, description, …
Section: sectionId, semester, professor name,
Student: studentId, name, major, …
Enrollment: studentID, sectionID
Room: building name, room number, room capacity
- or add these attributes to section

45. Complete Logical Model
Add
Primary keys
Foreign keys
Constraints
Domains
etc