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Topic
Database Modelling
Advanced Databases
S McKeever
3 phases of database design
Need a systematic approach to designing and building the dB
(1) Conceptual
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(2) Logical
S McKeever
(3) Physical
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Phase 1: Conceptual database design
Step 1: Build conceptual data model
– Step 1.1 Identify entity types
– Step 1.2 Identify relationship types
– Step 1.3 Identify and associate
attributes with entity or relationship
types
– Step 1.4 Determine attribute
domains
– Step 1.5 Determine candidate,
primary, and alternate key attributes
Advanced Databases
S McKeever
– Step 1.6 Consider use of
enhanced modeling
concepts (optional step)
– Step 1.7 Check model for
redundancy
– Step 1.8 Validate
conceptual model against
user transactions
– Step 1.9 Review
conceptual data model with
user
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Phase 2: Logical database design
Step 2: Build and validate Logical database
design
– Step 2.1 Derive relations for logical data
model
– Step 2.2 Validate relations using
normalization
– Step 2.3 Validate relations against user
transactions
– Step 2.4 Define integrity constraints
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S McKeever
– Step 2.5 Review logical
data model with user
– Step 2.6 Merge logical
data models into global
model (optional step)
– Step 2.7 Check for future
growth
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Phase 3: Physical database design
• Step 3 Translate logical data
model for target DBMS
Step 3.1 Design base relations
Step 3.2 Design representation of
derived data
Step 3.3 Design general constraints
Advanced Databases
S McKeever
Step 4 Design file
organizations and indexes
Step 4.1 Analyze transactions
Step 4.2 Choose file
organization
Step 4.3 Choose indexes
Step 4.4 Estimate disk space
requirements
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Phase 3: Physical database design
(continued)
• Step 5 Design user views
• Step 6 Design security mechanisms
• Step 7 Consider the introduction of controlled
redundancy
• Step 8 Monitor and tune the operational system
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Our focus…
• Will concentrate on the techniques for producing the
Entity Relationship Diagram (ERD)
• This is the typical design diagram for explaining
• The entities
• Their attributes
• Their relationships
• Key output from Conceptual data modelling
• Greater detail as logical and physical design
progresses,…
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Entities
• “An entity is a business object that represents
a group, or category of data.”
• Like a database table
Entity =
“thing”
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Entities diagramatically
Customer entity
Customer branches
(a customer can be in more than
One branch)
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Advanced Databases
S McKeever
Entities as table - with data
Customer
CustID
CustName
CustAddr
Credit_Lim
Curr_Bal
Dunn0001
Dunnes
Stores
Main
Street,
Dublin
£1000
£15.45
Customer branches
CustID
BranID
BranName
BranAddr
Credit_Lim
Curr_Bal
Dunn0001
RATH0001
Rathmines
Dublin 6
£200
£0.40
Dunn0001
JERV0001
Jervis St.
Dublin 2
£300
£6.05
Dunn0001
DAWS000
1
Dawson
St.
Dublin 2
£300
£6.00
Dunn0001
WICK0001
Wicklow
Wicklow
£200
£3.00
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So what does that mean ?
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S McKeever
• The DB is recording information about your
customers and their branches
• These are examples of real-world entities
• An entity is any “object” about which we wish
to store information in the DB (table)
• Entities are linked together by relationships
• CustID, BranAddr, etc. are data items or
fields or attributes
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Advanced Databases
S McKeever
To set up that table in SQL...
Create table Customer
( CustID char(10) NOT NULL,
CustName char(50),
CustAddr char(100),
Credit_Lim number(10),
Debtor_Bal number(10)
);
select * from Customer
where CustID = “Dunn0001”
Attributes
• “An attribute is a sub-group of information
within an entity.”
• Like a database field
• Shown as list under the entity name
Attributes =
“ways to
describe the
thing”
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Keys
Customer
CustID
CustName
CustAddr
Credit_Lim
Curr_Bal
Dunn0001
Dunnes
Stores
Main
Street,
Dublin
£1000
£15.45
Primary key = uniquely identifies EACH row
What’s the primary key here?
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What’s the primary key here?
(can be composite.. More than one field
Customer branches
CustID
BranID
BranName
BranAddr
Credit_Lim
Curr_Bal
Dunn0001
RATH0001
Rathmines
Dublin 6
£200
£0.40
Dunn0001
JERV0001
Jervis St.
Dublin 2
£300
£6.05
Dunn0001
DAWS000
1
Dawson
St.
Dublin 2
£300
£6.00
Dunn0001
WICK0001
Wicklow
Wicklow
£200
£3.00
Keys
Foreign key = A column(s) one a table that matches the
primary key on another table.
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CustID
CustName
CustAddr
Credit_Lim
Curr_Bal
Dunn0001
Dunnes
Stores
Main
Street,
Dublin
£1000
£15.45
CustID
BranID
BranName
BranAddr
Credit_Lim
Curr_Bal
Dunn0001
RATH0001
Rathmines
Dublin 6
£200
£0.40
Dunn0001
JERV0001
Jervis St.
Dublin 2
£300
£6.05
Dunn0001
DAWS000
1
Dawson
St.
Dublin 2
£300
£6.00
Dunn0001
WICK0001
Wicklow
Wicklow
£200
£3.00
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Keys
Advanced Databases
S McKeever
Foreign keys – enforce referential integrity
What happens if I (try) to delete customer DUNN001
from the customer table?
CustID
CustName
Dunn0001
Dunnes
Stores
X
CustAddr
Credit_Lim
Curr_Bal
Main
Street,
Dublin
£1000
£15.45
CustID
BranID
BranName
BranAddr
Credit_Lim
Curr_Bal
Dunn0001
RATH0001
Rathmines
Dublin 6
£200
£0.40
Dunn0001
JERV0001
Jervis St.
Dublin 2
£300
£6.05
Dunn0001
DAWS000
1
Dawson
St.
Dublin 2
£300
£6.00
Dunn0001
WICK0001
Wicklow
Wicklow
£200
£3.00
What does each key mean?
Primary key = uniquely identifies EACH row e.g.
student ID – everyone has a different one
+
Alternate key – could have been a primary key but
isn’t. e.g. student ID (PK) and PPS ID (AK)
=
Candidate keys: The primary key and the alternative keys. Any
column or a combination of columns that can qualify as unique
key in database.
e.g. student ID and PPS number
Foreign key = A column(s) one a table that matches the
primary key on another table.
E.g. student_school (on student table, links to table contains
schools)
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Entity attribute display in data models
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Entities and tables
The words “Entities “ and “tables” are sometimes used
interchangeably.
But they’re not quite the same thing….
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Relationships between entities
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To start defining relationships between entities, first describe the
way that they are connected in real life
Let’s go back to the ERD in the quiz
There are four faculties in the college: science, applied arts,
built environment, business. Each student is registered on a
programme within a faculty. Programmes consist of a set of
modules, with some module offered on more than one
programme. A student on a programme has to register for the
compulsory modules on tphe programme and also take a number
of optional modules. All modules have a unique module ID, and
an associated credit waiting (ECTS).
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Advanced Databases
S McKeever
Relationships between entities
Need to be able to say “how many” of the entities
Are participating in the relationship
1:1? One to one
1:M? One to many
N:M? Many to many
Sample Entity Relationship Diagram – book store
Note: The attributes are
listed
The data types of the
attributes are listed
What stage of the design
is this? Conceptual, logical
or physical?
Imagine you’ve drawn this ERD
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What was the business description of the data requirements given in the
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first place?
Book store ERD
Imagine you’ve drawn this ERD
What was the business description of the data requirements given
in the first place?
Some thing like –
Need to store information about the book titles that we sell,
In our bookstore, we store information about authors, the titles that
they write . An author can write many titles. If the author has died
they will have an obituary. The bookstore has many ongoing sales
promotions that include many titles
We categorise our titles along the usual lines – popular fiction, crime,
health and so on
We also store information about our publishers and who the main
sales contact is in the publisher
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Relationships between entities
When two entities are “related”.. There should be a word to describe this relationship
e.g.
Entities
staff, branches
student, modules
newspaper, rentalProperties
Relationship
Staff manage bank branches
student attends classes
newspapers advertise rentalProperties
You need to understand this “word” in order to figure out the information needed to define
the relationship for the ERD
On an ERD, need to know how many of each entity takes part in the relationship –
question both entities about “how many”?
e.g. For the Staff manages bank branches relationship
Staff can manage how many branches at most?? A staff person can manage one
branch
Staff must manage how many branches at min? A staff person can have no branch to
manage
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Relationship cardinality
“eh…What’s the
cardinality of your
mobile phone spend
in euro each week?
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Relationship
cardinality is about
assigning the 1:1, 1:n
etc etc
S McKeever
“Can” and “Must”
get you there..
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Maximum Cardinality
Can….
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The 3 types of maximum cardinality
One to one 1:1
One to many 1:n
Many to many m:n
An employee can be assigned one computer
A mother can have many children
An order can have many product and vv
How can you tell the difference between 1:m and m:n
e.g. Student and module entities. What’s the maximum cardinality?
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Minimum cardinality
Must…
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Minimal cardinality
What’s the minimum cardinality* for our examples?
An employee must be assigned ? computer
A mother must have ? children
An order must have ? product
* zero (optional) or one (must have at least one)
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Minimal cardinality
What’s the minimum cardinality* for our examples?
An employee must be assigned ? Computer
Ans: Depends on the rule of the individual company. Probably zero
A mother must have ? children
ans: Definitely zero (optional!)
An order can have ? Products
ans: must have at least one
* zero (optional) or one (must have at least one)
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Maximum and minimum cardinality
Now we know how to get the maximum (can) and minimum –
But it has to be done BOTH ways to complete the relationship.
We only did ONE direction
Maximum cardinality
Minimim cardinality
A computer can be assigned to one employee A computer must be assigned to one employee -No
A child can have one mother
A child must have a mother? YES
A product can be on many orders
A product must be on an order? NO
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Maximum and minimum cardinality
Once Maximum and Minimum cardinality has been figures out
Between the entities – then they can be notated in whatever design
Tool you’re using
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Just for reference - Data modelling notation:
Erwin
Max
Max
Min
Min
Note: Solid line ____ versus Dashed line ---- in Erwin = identifying versus non identifying
relationship between two entities
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Create relationships:
n:m relationships – intersection table
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Drawing the ERD with what you’ve learnt so
far…
Read the specification and make sure your understand it
Figure out what your entities are e.g. sales, book title, authors
Figure out which ones are related (don’t worry yet about the
cardinality – helps to put the word in (e.g. “has”)
Figure out the attributes of each entity
Identify the primary key for each entity
Any foreign keys?
Identify the cardinality of the relationship
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Exercise
A furniture company needs to develop a database to store
information about its customers and sales
Customer details need to be captured, such as name, address.
The company wants to be able to report on what their sales – what
each customer bought (i.e. what furniture items). They would also
like to know what the items are.
Try and sketch out the entities involved.
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Recursive relationships
EMPLOYEE
is supervised by
supervises
For reference.. Summary of Erwin
symbols
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Summary of Erwin symbols (cont.)
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Some examples
Note.. The symbols right beside the entity tell you how many of
“it” can and must apply to the other entity.
How many companies can and must an employee belong to?
(note: sometimes I I shown, sometimes I
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Some examples (continued)
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Some examples (continued)
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