Post on 30-Dec-2015
description
Relationships
LO
Today you are Learning
1. Understand the need for a relational database structure
2. Be able to create ERDs of the relationship
3. Use software to model and create a relational database
10/03/2014
R.D.B.M.S - Entities
• A relational database consists of multiple tables joined (linked) together by relationships
• Each table should contain data about a single entity– Person – Employee– Object - Book– Event - Project
R.D.B.M.S - Attributes
• Attributes describe the properties of an entity
• It’s easiest to think of attributes as fields
Relationships
ERDs
Entity relationship diagram
Relationships
• One to one 1:1
• One to many 1:M
• Many to Many M:M
Relationships
• One to one 1:1
• Teacher - classrooms
• One to many 1:M
• Teacher - courses
• Many to Many M:M
• Teachers - Students
Relationship?
Money
Relationships.....
Relationships.....
Draw the relationship shown below
• One to one 1:1
• Teacher - classrooms
• One to many 1:M
• Teacher - courses
• Many to Many M:M
• Teachers - Students
Worked Example 1
• Look at the iTunes database on the next slide
• Draw an ERD of the entities you would expect to see if this was converted into a relational database
HINT: There are 4 entities. All relationships are one-to-many
Worked Example 2
Worked Example 3
• Did you get something like this?
Album NameArtists Track Name
Genres
LO
Creating a relationship
• Load up the Actors database from the Cambridge Tech website
• Create a ‘Oscars’ Table to join the data
ENFORCE REFERENTIAL INTEGRITY
• An enforced relationship ensures each value entered in a foreign key column matches an existing value in the related primary key column.
Cascade Update Fields
• cascading updates only update existing records so that if you were to make a change to the customers table, all instances of the customer id would be changed in the customer orders table.