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. 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

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

6. The Attributes of the STUDENT Entity

7. Domains Attributes have domain Attributes may share a domain
Domain is attribute’s set of possible values
Attributes may share a domain

8. 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

9. Composite Primary Keys (continued)
Primary Keys (CLASS_CODE) Another possible Composite Primary Key (CRS_CODE + CLASS_SECTION)

10. 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

11. Classes of Attributes Multivalued attributes can have many values.
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.

12. Multivalued attributes in an Entity

13. Resolving Multivalued Attribute Problems
Although conceptual model can handle M:N relationships and multivalued attributes, you should not implement them in relational DBMS.
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).

14. Splitting the Multivalued Attribute into New Attributes

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

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

17. Derived Attributes 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.

18. Depiction of a Derived Attribute

19. Derived Attributes (continued)

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

21. 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

22. Connectivity
The term connectivity is used to describe the relationship classification (e.g., one-to-one, one-to-many, and many-to-many).

23. 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.

24. Connectivity and Cardinality in an ERD

25. Connectivity and Cardinality

26. 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

27. Relationship Strength
Weak (non-identifying) relationship
One entity is not existence-independent on another entity
exist if the PK of the related entity doesn’t contain a PK component of the parent entity
COURSE( CRS_CODE, …)
CLASS( CLASS_CODE, …)
Strong (identifying) relationship
exist when the related entities are existent-dependent and the PK of the dependent entity contains a PK component of the parent entity
CLASS( CRS_CODE, CLASS_SECTION, …)
一樣都是existence-dependent
但要分辨出 existence-dependent 的強弱程度

28. A Weak Relationship Between COURSE and CLASS

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

30. 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.

31. 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

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

33. 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

34. Relationship Participation
Optional participation
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 participation
One entity occurrence requires corresponding occurrence in related entity
If no optionality symbol is shown on ERD, it is mandatory

35. Relationship Participation (continued)
CLASS is Optional to PROFESSOR
PROFESSOR is Mandatory to CLASS

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

37. Relationship Participation (continued)
CLASS is Optional to COURSE
COURSE is Mandatory to CLASS

38. Relationship Participation (continued)
COURSE and CLASS in a Mandatory Relationship

39. Relationship Participation (continued)