1. Entity Relation Model
Tingting Zhang

2. ER Model Developed by P. Chen in 1976
One of the powerful uses of ER modeling is to assure that the data model reflects the user requirements.

3. Entity An entity is a thing that has an independent existence Ex:
Mittuniversity
ITM
L111
DATB03

4. Entity types
Define a set of entities of the same type (share the same structure)
EX:
Entity type Entity
University Mittuniversity
Department ITM
Class room L111
Course DATB03

5. Attributes An entity is described by its attributes. EX: Name E-mail
ID #
Address
Telephone #

6. Attributes Domain – the set of permitted values for each attribute
Attribute types:
Simple and composite attributes.
Single-valued and multi-valued attributes
E.g. multivalued attribute: phone-numbers
Derived attributes
Can be computed from other attributes
E.g. age, given date of birth

7. Composite Attributes

8. Attributes in ER diagrams
Simple single value attribute
Simple multi valued attribute
Composite Attributes
Birth date
telephones
name
Last name
First name

9. Key
An entities of an enetity type usually has an or set of attributes whoes values are distnct for each individual entity in the entity type.
Key in ER diagram
Department name

10. Null values Null: an unknown value or empty value Used when
Do not know the value of an attributed for a particular entity.
A particular entity may not have an applicable value for an attribute.

11. Entity Type in ER model
Name
ID#
telephone
Customer
address

12. Relation ship A relationship is an association among several entities
Example:
Hayes borrower A-102 customer loan
entity relationship entity

13. Relationship type
A relationship type among n entity E1, …, En defines a relationship set.
The relationship set is a set of relationship r among n  2 entities, each taken from entity sets
{(e1, e2, … en) | e1  E1, e2  E2, …, en  En} where (e1, e2, …, en) is a relationship

14. Relationship Type in ER diagrams

15. Relationship degree (I)
One to one
One to many

16. Relationship degree in ER diagrams (I.1)
address
name
Manage 1 1
Manager
Department
address
name
ID#

17. Relationship degree in ER diagrams (I.2)1 M

18. Relationship degree (II)
Many to one
Many to many

19. Relationship degree in ER diagrams (II.1)
Employee
department
Works-for M 1
ID#
address
name
location

20. Relationship degree in ER diagrams (II.2)
Employee
Project
Works-On M N
ID#
address
name
description

21. Roles
Each entity type that participates in a relationship type plays a particular role in the relationship.
Role name signifies the role that a participating entity from the entity type plays in each relationship instance
Roles are indicated in E-R diagrams by labeling the lines that connect diamonds to rectangles.
Role labels are optional, and are used to clarify semantics of the relationship

22. Roles in ER diagrams

23. Constraints on Relationship types Cardinality ratios for binary relationships
Employee
department
Works-for M 1
Cardinality ratios:
Employee
department
Works-for
1:1
4:N

24. Participation of an Entity Set in a Relationship Set
Total participation (indicated by double line): every entity in the entity set participates in at least one relationship in the relationship set
E.g. participation of loan in borrower is total
every loan must have a customer associated to it via borrower
Partial participation: some entities may not participate in any relationship in the relationship set
E.g. participation of customer in borrower is partial

25. Constraints on Relationship types Participation in ER diagram

26. Cardinality ratios for binary relationships
1:1

27. Attributes of relationship types
Relationship types can have attributes
Employee
project
Works-on M N
Hours per week
ID#
address
name
description

28. Weak entity (I)
An entity set that does not have a primary key is referred to as a weak entity set.
The existence of a weak entity set depends on the existence of a identifying entity set
it must relate to the identifying entity set via a total, one-to-many relationship set from the identifying to the weak entity set
Identifying relationship depicted using a double diamond

29. Weak entity (II)
The partial key of a weak entity set is the set of attributes that distinguishes among all the entities of a weak entity set.
The primary key of a weak entity set is formed by the primary key of the strong entity set on which the weak entity set is existence dependent, plus the weak entity set’s discriminator.

30. Weak entity (III) M Employee Dependents-of dependent
address
ID#
First name M 1
Employee
Dependents-of
dependent
Birth date
First name
Last name
The partial key is first name

31. An example database application
The company is organized into departments. Each department has a unique name, a unique number, and a particular employee who manages the department. We keep track of the start date when that employee began managing the department. A department may have several locations.
A department controls a number of projects, each of which has a unique name, a unique number, and a single location
We store each employee’s name, social security number, address, salary, sex and birth date. An employee is assigned to one department but may work on several projects, which are not necessarily controlled by the same department. We keep track of the number of hours per week that an employee works on each project. We also keep track of the direct supervisor of each employee.
We want to track of the dependents of each employee for insurance purposes. We keep each dependent’s first name, sex birth date, and relationship to the employee.