What is a database?
TDDD37 Database technology
Lecture 3
Enhanced Entity-Relationship (EER) Modeling
Jose M. Peña
[email protected]
Basic Definitions
Database:
Data:
Mini-world:
Database System Environment
A collection of related data.
Known facts that can be recorded and have an implicit meaning.
Some part of the real world about which data is stored in a database.
For example, student grades and transcripts at a university.
Database Management System (DBMS):
A database represents some aspect of the
real world, i.e. a mini world.
A database consists of a logical coherent
collection of data with an underlying
meaning.
A database is designed, built and filled
with data with respect to an underlying
purpose.
A software package/ system to facilitate the creation and maintenance
of a computerized database.
Database System:
The DBMS software together with the data itself. Sometimes, the
applications are also included.
Database Design Process
Overview
Real world
Model
Database
DBMS
Query
Answer
Processing of
queries and updates
Access to stored data
Physical
database
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Entity-Relationship (ER) Model
Entity and entity type
High-level conceptual data model
An
overview of the database
to discuss with non-database experts
Easy to translate to data model of DBMS
Easy
Entity – a ”thing” in the real world with an
independent existence
Attributes – properties that describes an entity
Entity type – a collection of entities that have the
same set of attributes
Model
RegNumber
ER diagram
Year
PersonalNumber
Name
Car
Owner
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Attributes
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Constraints on attributes
Simple vs composite
Single-valued vs multivalued
Stored vs derived
Value sets (domains) of attributes
Key attributes
Street
Street
City
Name
City
Address
Name
PersonalNumber
Address
PersonalNumber
Owner
Owner
PhoneNumber
Age
PhoneNumber
Age
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Relationship type
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Constraints on relationship types
Relationship type – association among entity types
Cardinality ratio – maximum number of
relationship instances that an entity can
participate in
possible cardinality ratio: 1:1, 1: N, N:1, N:M
Model
RegNumber
PersonalNumber
Name
owns
N
Owner
Car
1
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Car
owns
N
Owner
Car
owns
N
N
1
Owner
1
Owner
Year
M
owns
Car
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Constraints on relationship types
Constraints on relationship types
Participant constraint
Total
participation – an entity must exist related to
another entity
A weak entity can be identified uniquely by being
related to another entity (together with its own
attributes).
”Every car must be owned by at least one owner.”
PersonalNumber
N
M
Owner
Weak entity types– do not have key attibutes
of their own.
RegNumber
Car
owns
name
Name
number
N
1
team
players
Plays_on
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Attributes of relationship types
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N-ary relationships
”Store information on who owned which car and during
which period of time”
Example. A person works as an engineer at one
company and as a gym instructor at another
company.
Company
N
SellDate
Name
RegNumber
BuyDate
Employee
works at
Model
N
M
JobType
works as
M
Year
Employee
K
Ternary
N
PersonalNumber
N
Owner
works as
M
owns
Car
Company
M
JobType
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ER Notation
Symbol
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Enhanced ER (EER) Model
Meaning
ENTITY TYPE
WEAK ENTITY TYPE
RELATIONSHIP TYPE
Why more?
IDENTIFYING RELATIONSHIP TYPE
More complex data requirements
ATTRIBUTE
KEY ATTRIBUTE
MULTIVALUED ATTRIBUTE
Example. Only some employees can use a company car, only
managers have to write a monthly report, but all employees
have assigned personal number, salary account and a place in
the office.
COMPOSITE ATTRIBUTE
DERIVED ATTRIBUTE
E1
E1
R
1
R
N
E2
TOTAL PARTICIPATION OF E2 IN R
E2
CARDINALITY RATIO 1:N FOR E1:E2 IN R
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Subclass/superclass,
specialization/generalization,
union/category
attribute and relationship inheritance
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FirstName
Name
Surname
PN
d
U
Manager
Commission
ProjectLeader
Salesman
U
Engineer
1
U
U
U
Salesman
o
U
o
U
d
FirstName
Name
Employee
process of
defining
classes
Employee
Engineer
Manager
writes
1
U
uses
N
Car
MonthlyReport
PID
RegNumber
ReportID
Software
ProjectLeader
1
N
SoftwareProject
manages
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Example 3
A taxi company needs to model their activities
Union/category
ProjectLeader
1
U
Commission
Single vs. Multiple inheritance
U
specialization
Surname
PN
generalization
Subclass/Superclass
A subclass represents a collection of entities that is a
subset of the UNION of the entities of multiple distinct
superclasses
CNumber
PersonalNumber
Person
Address
Company
“An owner of a car is
either a person or a company.”
BirthDate
u
U
1
Owner
N
owns
Car
There are two types of employees in the company: drivers
and operators. For drivers it is interesting to know the date
of issue and type of the driving license, and the date of
issue of the taxi driver’s certificate. For all employees it is
interesting to know their personal number, address and the
available phone numbers.
The company owns a number of cars. For each car there is
a need to know its type, year of manufacturing, number of
places in the car and date of the last service.
The company wants to have a record of car trips
(körningar). A taxi may be picked on a street or ordered
though an operator who assigns the order to a certain
driver and a car. Departure and destination addresses
together with times should also be recorded.
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TaxiCertifDate
Street
PN
Address
Phone
PostNumber
A driver may have many
driving licenses (types)
DrivingLicenseDate
Driver
Town
Employee
DrivingLicenseType
o
DrivingLicenseDate
U
U
TaxiCertifDate
Driver
1
DrivingLicenseType
Date
Operator
Type
Id
1
TaxiCertifDate
N
assign
drives
1
belongsTo
DrivingLicense
Körkort
Driver
N
DepTime
Destination
Type
N
RegNumber
ServiceDate
DestTime
ID
1
N
belongsTo
M
Driver
Places
Car
Date
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Type
DrivingLicense
N
made_by
TaxiCertifDate
YearOfManuf
Trip
DeparturePlace
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Summary
Entity-Relationship (ER) diagram – a
graphical way to model the world
Main concepts - entity, relationship and
attribute
Different types of constraints
Enhanced ER model
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