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Data Modeling Using the
Entity-Relationship (ER) Model
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Example COMPANY Database
We need to create a database schema design based on the following (simplified) requirements of the COMPANY Database:
The company is organized into DEPARTMENTs. Each department has a name, number and an employee who manages the department. We keep track of the start date of the department manager. A department may have several locations.
Each department controls a number of PROJECTs. Each project has a unique name, unique number and is located at a single location.

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Example COMPANY Database (Continued)
The database will store each EMPLOYEE’s social security number, address, salary, sex, and birthdate.
Each employee works for one department but may work on several projects.
The DB will keep track of the number of hours per week that an employee currently works on each project.
It is required to keep track of the direct supervisor of each employee.
Each employee may have a number of DEPENDENTs.
For each dependent, the DB keeps a record of name, sex, birthdate, and relationship to the employee.

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ER Model Concepts
Entities and Attributes
Entity is a basic concept for the ER model. Entities are specific things or objects in the mini-world that are represented in the database.
For example the EMPLOYEE , the Research DEPARTMENT, the ProductX PROJECT
Attributes are properties used to describe an entity.
For example an EMPLOYEE entity may have the attributes Name, SSN, Address, Sex, BirthDate
A specific entity will have a value for each of its attributes.
For example a specific employee entity may have Name=’ ‘, SSN=’123456789′, Address =’731, Fondren, Houston, TX’, Sex=’M’, BirthDate=’09-JAN-55‘
Each attribute has a value set (or data type) associated with it – e.g. integer, string, date, enumerated type, …

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Types of Attributes (1)
Each entity has a single atomic value for the attribute. For example, SSN or Sex.
The attribute may be composed of several components. For example:
Address(Apt#, House#, Street, City, State, ZipCode, Country), or
Name(FirstName, MiddleName, LastName).
Composition may form a hierarchy where some components are themselves composite.
Multi-valued
An entity may have multiple values for that attribute. For example, Color of a CAR or PreviousDegrees of a STUDENT.
Denoted as {Color} or {PreviousDegrees}.

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Example of a composite attribute

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Entity Types and Key Attributes (1)
Entities with the same basic attributes are grouped or typed into an entity type.
For example, the entity type EMPLOYEE and PROJECT.
An attribute of an entity type for which each entity must have a unique value is called a key attribute of the entity type.
For example, SSN of EMPLOYEE.

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Entity Types and Key Attributes (2)
A key attribute may be composite.
VehicleTagNumber is a key of the CAR entity type with components (Number, State).
An entity type may have more than one key.
The CAR entity type may have two keys:
VehicleIdentificationNumber (popularly called VIN)
VehicleTagNumber (Number, State), aka license plate number.
Each key is underlined (Note: this is different from the relational schema where only one “primary key is underlined).

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Displaying an Entity type
In ER diagrams, an entity type is displayed in a rectangular box
Attributes are displayed in ovals
Each attribute is connected to its entity type
Components of a composite attribute are connected to the oval representing the composite attribute
Each key attribute is underlined
Multivalued attributes displayed in double ovals
See the full ER notation in advance on the next slide

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NOTATION for ER diagrams

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Initial Design of Entity Types:
EMPLOYEE, DEPARTMENT, PROJECT, DEPENDENT

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Refining the initial design by introducing relationships
The initial design is typically not complete
Some aspects in the requirements will be represented as relationships
ER model has three main concepts:
Entities (and their entity types and entity sets)
Attributes (simple, composite, multivalued)
Relationships (and their relationship types and relationship sets)
We introduce relationship concepts next

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Relationships and Relationship Types (1)
A relationship relates two or more distinct entities with a specific meaning.
For example, EMPLOYEE works on the ProductX PROJECT, or EMPLOYEE Franklin Wong manages the Research DEPARTMENT.
Relationships of the same type are grouped or typed into a relationship type.
For example, the WORKS_ON relationship type in which EMPLOYEEs and PROJECTs participate, or the MANAGES relationship type in which EMPLOYEEs and DEPARTMENTs participate.
The degree of a relationship type is the number of participating entity types.
Both MANAGES and WORKS_ON are binary relationships.

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Relationship instances of the WORKS_FOR N:1 relationship between EMPLOYEE and DEPARTMENT

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Relationship instances of the M:N WORKS_ON relationship between EMPLOYEE and PROJECT

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ER DIAGRAM – Relationship Types are:
WORKS_FOR, MANAGES, WORKS_ON, CONTROLS, SUPERVISION, DEPENDENTS_OF

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Constraints on Relationships
Constraints on Relationship Types
(Also known as ratio constraints)
Cardinality Ratio (specifies maximum participation)
One-to-one (1:1)
One-to-many (1:N) or Many-to-one (N:1)
Many-to-many (M:N)
Existence Dependency Constraint (specifies minimum participation) (also called participation constraint)
zero (optional participation, not existence-dependent)
one or more (mandatory participation, existence-dependent)

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Many-to-one (N:1) Relationship

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Many-to-many (M:N) Relationship

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UML class diagrams
Represent classes (similar to entity types) as large rounded boxes with three sections:
Top section includes entity type (class) name
Second section includes attributes
Third section includes class operations (operations are not in basic ER model)
Relationships (called associations) represented as lines connecting the classes
Other UML terminology also differs from ER terminology
Used in database design and object-oriented software design
UML has many other types of diagrams for software design

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UML class diagram for COMPANY database schema

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