Post on 23-Feb-2016
description
Topic Database Modelling
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3 phases of database design
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(1) Conceptual (2) Logical (3) Physical
Need a systematic approach to designing and building the dB
Phase 1: Conceptual database design
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– 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
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– 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
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Step 3.1 Design base relationsStep 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)
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• 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…
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• 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
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Entity = “thing”
• “An entity is a business object that represents a group, or category of data.”
•Like a database table
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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
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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
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CustID CustName CustAddr Credit_Lim Curr_Bal
Dunn0001 DunnesStores
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
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
Customer branches
Keys
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Foreign key = A column(s) one a table that matches the primary key on another table.
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
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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
KeysForeign keys – enforce referential integrityWhat happens if I (try) to delete customer DUNN001 from the customer table?
X
What does each key mean?
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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
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Entities and tables
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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
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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
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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
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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
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“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
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Can….
The 3 types of maximum cardinality
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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
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Must…
Minimal cardinality
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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)
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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
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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
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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
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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
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Drawing the ERD with what you’ve learnt so far…
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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
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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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EMPLOYEE supervises
is supervised by
Recursive relationships
For reference.. Summary of Erwin symbols
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Summary of Erwin symbols (cont.)
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Some examples
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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)
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Some examples (continued)
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