COIS 342 - Databases
Chapter 3
Data Modeling Using the Entity-
Relationship (ER) Model
Faculty of Computing and Information Technology in Rabigh
Adapted from Elmasri & Navathe
by Dr Samir BOUCETTA
First Semester 2011/2012
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COIS 342 - Databases
Chapter 3
Data Modeling Using the Entity-
Relationship (ER) Model
F aculty of C omputing and I nformation T echnology in R abigh
Adapted from Elmasri & Navathe
by Dr Samir BOUCETTA
First Semester 2011/
Example COMPANY
Database
Requirements of the Company (oversimplified for
illustrative purposes)
– 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.
– Each department controls a number of PROJECTs.
Each project has a name, number and is located at a
single location.
ER Model Concepts Entities and Attributes
- Entities are specific objects or things in the mini-world that are represented in the database. For example the EMPLOYEE “Mohammad Ali”, the “Research and Development” DEPARTMENT, the “Bridge construction” PROJECT
- Attributes are properties used to describe an entity. For example an EMPLOYEE entity may have a Name, SSN, Address, Sex, …
- A specific entity will have a value for each of its attributes. For example a specific employee entity may have Name=„Mohammad Ali', SSN=' 123456789 ', Address =„71, King Fahd Street, Jeddah', Sex='M', BirthDate='09-JAN-1955„
- Each attribute has a value set (or data type) associated with it – e.g. integer, string, subrange, enumerated type, …
Types of Attributes (1) Simple
- Each entity has a single atomic value for the attribute. For example, SSN or Sex. Composite
- The attribute may be composed of several components. For example, Address (Appart#, 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}.
Entity Types and Key Attributes Entities with the same basic attributes are grouped or typed into an entity type. For example, the EMPLOYEE entity type or the PROJECT entity type. 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. A key attribute may be composite. For example, VehicleTagNumber is a key of the CAR entity type with components (Number, State). An entity type may have more than one key. For example, the CAR entity type may have two keys:
- VehicleIdentificationNumber (popularly called VIN) and
- VehicleTagNumber (Number, State), also known as license_plate number.
ENTITY SET corresponding to the ENTITY TYPE CAR car 1 ((ABC 123, TEXAS), TK629, Ford Mustang, convertible, 1999, (red, black)) car 2 ((ABC 123, NEW YORK), WP9872, Nissan 300ZX, 2-door, 2002, (blue)) car 3 ((VSY 720, TEXAS), TD729, Buick LeSabre, 4-door, 2003, (white, blue)) . . . CAR Registration(RegistrationNumber, State), VehicleID, Make, Model, Year, (Color)
ER DIAGRAM – Entity Types are: EMPLOYEE, DEPARTMENT, PROJECT, DEPENDENT
Relationships and Relationship Types (1) A relationship relates two or more distinct entities with a specific meaning. For example, EMPLOYEE John Smith 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.
Example relationship instances of the WORKS_ON relationship between EMPLOYEE and PROJECT e 1 e 2 e 3 e 4 e 5 e 6 e 7 r 1 r 2 r 3 r 4 r 5 r 6 r 7 p 1 p 2 p 3 r 8 r 9
Relationships and Relationship Types (2) More than one relationship type can exist with the same participating entity types. For example, MANAGES and WORKS_FOR are distinct relationships between EMPLOYEE and DEPARTMENT, but with different meanings and different relationship instances.
Weak Entity Types
An entity that does not have a key attribute
A weak entity must participate in an identifying relationship
type with an owner or identifying entity type
Entities are identified by the combination of:
– A partial key of the weak entity type
– The particular entity they are related to in the
identifying entity type
Example:
Suppose that a DEPENDENT entity is identified by the
dependent‟s first name and birhtdate, and the specific
EMPLOYEE that the dependent is related to. DEPENDENT
is a weak entity type with EMPLOYEE as its identifying
entity type via the identifying relationship type
DEPENDENT_OF
Weak Entity Type is: DEPENDENT Identifying Relationship is: DEPENDENTS_OF
Many-to-one (N:1) RELATIONSHIP e 1 e 2 e 3 e 4 e 5 e 6 e 7 EMPLOYEE r 1 r 2 r 3 r 4 r 5 r 6 r 7 WORKS_FOR d 1 d 2 d 3 DEPARTMENT
Many-to-many (M:N) RELATIONSHIP e 1 e 2 e 3 e 4 e 5 e 6 e 7 r 1 r 2 r 3 r 4 r 5 r 6 r 7 p 1 p 2 p 3 r 8 r 9