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 – because these describe the entity

Discovering Attributes 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

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 Potential multi-valued attribute; therefore create a new entity

Discovering Attributes

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 attributes Identified 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 Course code could uniquely identify this entity and therefore is a candidate to be a primary key None of these attributes could uniquely identify a composer; therefore, need to create a new entity

Identifying Keys What should be the Primary Key for the following examples?

Identifying Keys Foreign Key Created automatically through relationships between entities An attribute whose value matches the primary key value in the related table

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

Entities translate into tables Attributes translate into fields Table is made up of columns (fields) and rows (records)

Relational Model CompositionIDTitleComposerID 111Rhapsody in BlueG18 222Second RhapsodyG18 333Academic Festival OvertureB Tragic OvertureB Ein deutsches RequiemB113 ComposerIDNameCountryYearOfBirthYearOfDeath B33Johann Sebastian BachGermany B76Ludwig van BeethovenGermany B90Irving BerlinAmerica B113Johannes BrahmsGermany G18George GershwinAmerica G80Orlando GibbonsEngland 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