dbs201: relational databases. agenda entity relationship model discovering attributes identifying...
TRANSCRIPT
DBS201: Relational Databases
Agenda
Entity Relationship Model Discovering Attributes Identifying Keys Defining Relationships Relational Model
Entity Relationship Diagram
Shows entities – similar to a table structure An entity is an object that exists and is
distinguishable from other objects An entity is represented by a set of attributes An attribute has a domain (i.e. a set of
permitted values) Entities are associated by relationships
Agenda
Entity Relationship Model Discovering Attributes Identifying Keys Defining Relationships Relational Model
Attributes
Attributes are characteristics of Entities. They describe an entity Different types of attributes:
Simple Composite Single-Valued Multi-Valued
Attributes have a Domain
Attributes
Simple An attribute that can not be divided i.e. FIRST_NAME
Attributes
Composite Attributes that can be further subdivided to yield
additional attributes i.e. NAME can be subdivided into FIRST_NAME,
INITIAL, and LAST_NAME
Attributes
Single-Valued An attribute that can have only a single value Not necessarily a simple attribute i.e. Customer Number, Part Number
Attributes
Multi-Valued An attribute that can have many values i.e. Education Two possible ways to deal with these:
Create attributes for each value Create a new entity composed of the original multi-valued
attributes components
Second approach is preferred
Remember requirements gathering approaches?
Different approaches: bottom-up/top-down Top-down uses an overview/narrative of a
system to discover entities Bottom-up uses screen/report samples to
discover attributes, then these are grouped into entities
Discovering Attributes
Top-down Interview client Look for description of entities (i.e. their
characteristics in a narrative)
Bottom-up Use fields on screen/report samples
Discovering Attributes
From this example:1. A composer writes/creates many
compositions.
2. The composer includes the name, country, year of birth, year of death and era (e.g. classical, baroque, romantic, or modern).
3. The composition includes a title, type of composition (symphony, concerto, instrumental, chamber, opera, or choral)
Discovering Attributes
attributes
attributes
Attributes – because these describe the entity
Discovering Attributes
Composer
Name Country YearOfBirth YearOfDeath
Composition
Title
Era
Description
CompositionType
Description
The composer includes the name, country, year of birth, year of death and era (e.g. classical, baroque, romantic, or modern).
The composition includes a title, type of composition (symphony, concerto, instrumental, chamber, opera, or choral)
No reference to era; assuming it is another entity
No to composition type; assuming it is another entity
Potential Attributes: Team
Title Player
Name, Address, DateOfBirth, Phone Number Sponsor
Name, Address, Phone Number, Fax Coaches
Name, Address, Phone Number District
Name, Description
Discovering Attributes
Discovering Attributes
Team
Title
Coach
Name Address PhoneNumber
Sponsor
Name Address PhoneNumber Fax
Player
Name Address DateOfBirth PhoneNumber
District
Name Description
From this example:Multiple Real Estate Listing Service (MRELS) is a company that has offices throughout Southern Ontario. These offices have a number of Real Estate agents working for them. MRELS deals in both residential and commercial real estate. Agents work with customers to get a listing or to find a listing for a customer. A listing will include the address and the type of property (residential or commercial), square footage, annual taxes and closing date. If it is a residential property, it will include number of bedrooms, number of bathrooms, special features (central air, heated by gas or oil, radiators or forced air, for instance). For a commercial property, it will include number of offices as well as number of docking bays (where trucks back up to be loaded/unloaded).
Discovering Attributes
Some attributes described – We will have to assume what some of the attributes might be
Discovering Attributes
Agent
Name PhoneNumber FaxNumber
Office
Name Address PhoneNumber
Customer
Name Address PhoneNumber
CommercialListing
Address SquareFootage AnnualTaxes ClosingDate NumberOfOffices NumberOfDockingBays
ResidentialListing
Address SquareFootage AnnualTaxes ClosingDate NumberOfBedrooms NumberOfBathrooms SpecialFeatures
Potential multi-valued attribute; thereforecreate a new entity
Discovering Attributes
Agent
Name PhoneNumber FaxNumber
Office
Name Address PhoneNumber
Customer
Name Address PhoneNumber
CommercialListing
Address SquareFootage AnnualTaxes ClosingDate NumberOfOffices NumberOfDockingBays
ResidentialListing
Address SquareFootage AnnualTaxes ClosingDate NumberOfBedrooms NumberOfBathrooms
SpecialFeatures
CentralAirFlag HeatingSystem
Agenda
Entity Relationship Model Discovering Attributes Identifying Keys Defining Relationships Relational Model
A key is used to identify and locate the information contained in a particular record
A key consists of one or more attributes that determine other attributes
A multi-attribute key is called a composite key
Two types of keys we will address: Primary Key and Foreign Key
Identifying Keys
Identified from original list of attributesIdentified through relationships
Identifying Keys
Primary Key Uniquely identifies any given entity
If an entity does not contain an attribute that would be a good Primary Key candidate, then need to create a new attribute to be the Primary Key
Composer
Name Country YearOfBirth YearOfDeath
Course code could uniquely identify this entity and therefore is a candidate to be a primary key
Course
CourseCode Name Description
None of these attributes could uniquely identify a composer; therefore, need to create a new entity
Course
PK CourseCode
Name Description
Composer
PK ComposerID
Name Country YearOfBirth YearOfDeath
Identifying Keys
What should be the Primary Key for the following examples?
Composition
Title
Era
Description
CompositionType
Description
Identifying Keys
Foreign Key Created automatically through relationships between
entities An attribute whose value matches the primary key
value in the related table
Course
PK CourseCode
Name Description
Section
PK SectionCode
MaxNumStudentsFK1 CourseCode
Agenda
Entity Relationship Model Discovering Attributes Identifying Keys Defining Relationships Relational Model
Defining Relationships
1:1 Determine most obvious path (i.e. which entity is
the true ‘1’). Take the PK from that true ‘1’ and make it an FK in the other ‘1’
1:M Take the PK from the 1 and make it an FK in the M
M:N Create a bridge table and take the PKs from each
original table and make the PKs and FKs of the bridge table
Agenda
Entity Relationship Model Discovering Attributes Identifying Keys Defining Relationships Relational Model
Relational Model
Entities translate into tables Attributes translate into fields Table is made up of columns (fields) and rows
(records)
Composer
PK ComposerID
Name Country YearOfBirth YearOfDeath
Composition
PK CompositionID
TitleFK1 ComposerID
Relational ModelComposer
PK ComposerID
Name Country YearOfBirth YearOfDeath
Composition
PK CompositionID
TitleFK1 ComposerID
CompositionID Title ComposerID
111 Rhapsody in Blue G18
222 Second Rhapsody G18
333 Academic Festival Overture B113
444 Tragic Overture B113
555 Ein deutsches Requiem B113
ComposerID Name Country YearOfBirth YearOfDeath
B33 Johann Sebastian Bach Germany 1685 1750
B76 Ludwig van Beethoven Germany 1770 1827
B90 Irving Berlin America 1888 1989
B113 Johannes Brahms Germany 1833 1897
G18 George Gershwin America 1898 1937
G80 Orlando Gibbons England 1583 1625
Primary keys are obvious – they uniquely identify a row.
Foreign keys are obvious – they refer to a field in another table.
Relational Model
FIGURE P3.23 Table name: TRUCK Database name: Ch03_TransCo
Table name: BASE
Table name: TYPE 1. What are the Primary Keys for each table?2. What are the Foreign Keys for each table?3. Draw the ERD