topic database modelling s mckeever advanced databases 1

Topic Database Modelling

S McKeeverAdvanced Databases

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3 phases of database design

S McKeeverAdvanced Databases 2

(1) Conceptual (2) Logical (3) Physical

Need a systematic approach to designing and building the dB

Phase 1: Conceptual database design


– 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

– 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

Step 1: Build conceptual data model

Phase 2: 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

– 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

Step 2: Build and validate Logical database design

Phase 3: Physical database design


Step 3.1 Design base relations

Step 3.2 Design representation of derived data

Step 3.3 Design general constraints

• Step 3 Translate logical data model for target DBMS

Step 4 Design file organizations and indexes

Step 4.1 Analyze transactionsStep 4.2 Choose file organizationStep 4.3 Choose indexesStep 4.4 Estimate disk space requirements

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

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,…

Entities


Entity = “thing”

• “An entity is a business object that represents a group, or category of data.”

•Like a database table

S McKeever

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Entities diagramatically

Customer branches

(a customer can be in more thanOne branch)

Customer entity


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CustID CustName CustAddr Credit_Lim Curr_Bal

Dunn0001 DunnesStores

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 DAWS0001

Dawson St.

Dublin 2 £300 £6.00

Dunn0001 WICK0001 Wicklow Wicklow £200 £3.00

Entities as table - with data

Customer

Customer branches


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•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

So what does that mean ?


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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 Customerwhere CustID = “Dunn0001”

Attributes


Attributes = “ways to describe the thing”

•“An attribute is a sub-group of information within an entity.”

•Like a database field•Shown as list under the entity name

Keys




Main Street,Dublin

£1000 £15.45

Customer

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




Dunn0001 DAWS0001

Dawson St.

Dublin 2 £300 £6.00


Customer branches

Keys


Foreign key = A column(s) one a table that matches the primary key on another table.



Main Street,Dublin

£1000 £15.45




Dunn0001 DAWS0001

Dawson St.

Dublin 2 £300 £6.00



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Main Street,Dublin

£1000 £15.45




Dunn0001 DAWS0001

Dawson St.

Dublin 2 £300 £6.00


KeysForeign keys – enforce referential integrityWhat happens if I (try) to delete customer DUNN001 from the customer table?

X

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)

Entity attribute display in data models


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 entitiesTo 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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Need to be able to say “how many” of the entitiesAre participating in the relationship

1:1? One to one1:M? One to manyN:M? Many to many

Relationships between entities

Sample Entity Relationship Diagram – book store


Note: The attributes are listedThe data types of the attributes are listedWhat stage of the design is this? Conceptual, logical or physical?

Imagine you’ve drawn this ERD

What was the business description of the data requirements given in the 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

Relationships between entities


When two entities are “related”.. There should be a word to describe this relationship

e.g. Entities Relationshipstaff, branches Staff manage bank branches student, modules student attends classesnewspaper, rentalProperties 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 branchStaff must manage how many branches at min? A staff person can have no branch to manage

Relationship cardinality


“eh…What’s the cardinality of your mobile phone spend in euro each week?

Relationship cardinality is about assigning the 1:1, 1:n etc etc

“Can” and “Must” get you there..

Maximum Cardinality


Can….

The 3 types of maximum cardinality


One to one 1:1 An employee can be assigned one computer

One to many 1:n A mother can have many children

Many to many m:n An order can have many product and vv

How can you tell the difference between 1:m and m:ne.g. Student and module entities. What’s the maximum cardinality?

Minimum cardinality


Must…

Minimal cardinality


An employee must be assigned ? computer

A mother must have ? children

An order must have ? product

What’s the minimum cardinality* for our examples?

* zero (optional) or one (must have at least one)


Minimal cardinality

An employee must be assigned ? ComputerAns: Depends on the rule of the individual company. Probably zero

A mother must have ? childrenans: Definitely zero (optional!)

An order can have ? Productsans: must have at least one

What’s the minimum cardinality* for our examples?

* zero (optional) or one (must have at least one)

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

A computer can be assigned to one employee

A child can have one mother

A product can be on many orders

Maximum cardinality

A computer must be assigned to one employee -No

A child must have a mother? YES

A product must be on an order? NO

Minimim cardinality

Maximum and minimum cardinality


Once Maximum and Minimum cardinality has been figures outBetween the entities – then they can be notated in whatever designTool you’re using

Just for reference - Data modelling notation:Erwin


Max

Min Min

Max

Note: Solid line ____ versus Dashed line ---- in Erwin = identifying versus non identifying relationship between two entities

Create relationships:n:m relationships – intersection table


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

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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EMPLOYEEsupervises

is supervised by

Recursive relationships

For reference.. Summary of Erwin symbols


Summary of Erwin symbols (cont.)


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

Some examples (continued)


topic database modelling s mckeever advanced databases 1

Documents

user step

indexes step

normalization step

logical data model step

entity types step

user transactions step

design user views step

design base relations