TDDB77
August 08
Overview
Real world
Query
Model
Data Modeling Using
ER/EER model
Database
Answer
Processing of
queries and updates
DBMS
He Tan
hetan@ida.liu.se
Access to stored data
Physical
database
2
Entity-Relationship (ER) Model
„
Main concepts
„
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 existance
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
3
Main concepts cont.
„
4
Attributes
Relationship type – association among entity types
„
„
„
Simple vs composite
Single-valued vs multivalued
Stored vs derived
Street
City
Model
RegNumber
PersonalNumber
Name
Name
Year
Address
PersonalNumber
N
1
Owner
owns
Car
Owner
PhoneNumber
Age
5
IDA
6
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TDDB77
August 08
Constraints on attributes
„
„
Example 1
Value sets (domains) of attributes
Key attributes
Street
Person lives at a certain address
„
City
Are addresses important in the database?
Name
Address
PersonalNumber
Owner
PhoneNumber
Age
7
Constraints on relationship types
Constraints on relationship types
„
8
„
Cardinality ratio – maximum number of
relationship instances that an entity can
participate in
Participant constraint
Total
participation – an entity must exist related to
another entity
possible cardinality ratio: 1:1, 1: N, N:1, N:M
”Every car must be owned by at least one owner.”
PersonalNumber
1
N
N
Owner
N
M
Owner
Car
owns
RegNumber
Car
owns
1
Owner
Car
owns
N
M
Owner
Car
owns
9
Constraints on relationship types
„
10
Attributes of relation types
Weak entity types– do not have key attibutes
of their own.
”Store information on who owned which car and during
which period of time”
an weak entity can be identified uniquely by being
related to another entity (together with its own
attributes).
SellDate
Name
Year
PersonalNumber
Name
version
N
1
Software
has_version
N
Owner
Software version
11
IDA
Model
RegNumber
BuyDate
M
owns
Car
12
2
TDDB77
August 08
N-ary relationships
ER Notation
Meaning
Symbol
ENTITY TYPE
WEAK ENTITY TYPE
„
Example. A person works as an engineer at one
company and as a gym instructor at another
company.
RELATIONSHIP TYPE
IDENTIFYING RELATIONSHIP TYPE
ATTRIBUTE
KEY ATTRIBUTE
Company
Employee
N
N
works at
M
JobType
MULTIVALUED ATTRIBUTE
works as
COMPOSITE ATTRIBUTE
M
K
Employee
Ternary
DERIVED ATTRIBUTE
N
Company
works as
E1
R
M
1
E1
JobType
R
N
E2
TOTAL PARTICIPATION OF E2 IN R
E2
CARDINALITY RATIO 1:N FOR E1:E2 IN R
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Example 2
Enhanced ER Model (EER)
Customers can rent video films
„
Why more?
More complex data requirements
Can the same video be rented more than
once?
„ Can a video exist without customers?
„ Is the rental history important?
„
„
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.
Subclass/superclass,
specialization/generalization,
union/category
attribute and relationship inheritance
Manager
U
Manager
U
writes
ProjectLeader
N
MonthlyReport
PID
ReportID
17
IDA
Engineer
U
RegNumber
Salesman
1
1
Car
Commission
ProjectLeader
o
U
uses
d
U
U
U
Engineer
1
FirstName
Name
Employee
o
U
d
Salesman
Surname
PN
process of
defining
classes
Employee
Commission
Single vs. Multiple inheritance
U
FirstName
Name
16
U
Surname
PN
specialization
Subclass/Superclass
generalization
15
SoftwareProject
N
1
manages
Software
ProjectLeader
18
3
TDDB77
August 08
Example 3
A taxi company needs to model their activities
Union/category
„
A subclass represents a collection of entities that is a
subset of the UNION of the entities of multiple distinct
superclasses
CNumber
PersonalNumber
Person
BirthDate
Address
“An owner of a car may be
a person or a company.”
Company
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.
19
20
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
DrivingLicenseType
Date
Operator
1
Type
Id
1
TaxiCertifDate
N
assign
drives
1
belongsTo
DrivingLicense
Körkort
Driver
N
DepTime
Destination
Type
N
RegNumber
ServiceDate
DestTime
ID
TaxiCertifDate
YearOfManuf
Trip
DeparturePlace
Type
DrivingLicense
N
made_by
1
N
belongsTo
M
Driver
Places
Car
Date
21
Summary
22
Overview
Real world
Entity-Relationship diagram – a graphical
way to model the world
„ Main concepts - entity, relationship and
attribute
„ Different types of constraints
„ Enhanced ER model
„
Model
Database
DBMS
Query
Answer
Processing of
queries and updates
Access to stored data
Physical
database
23
IDA
24
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