Database Design By Joysy John Code First Girls 15 Aug 2013
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Database Design
By Joysy John
Code First Girls 15 Aug 2013
Agenda
• Introduction
• Tables and Keys
• Types of Relationships
• Normalisation
• Q&A
Computer Engineer, MBA Founder, Social Entrepreneur Angel Investor, Ex-Banker, Mother
Education
Corporate & Entrepreneurial Career
Introduction
Tables and Keys
• Entities: An entity is a distinct clearly identifiable object of the database e.g. Book
• Attribute: Each Entity is characterised by a set of attributes e.g. Title
• Entity set: Set of all entities having attributes of the same type
• Relationships: A relationship is a mapping between entity sets
Book
Acc_No
Title
Author
YearofPub
Users
Card_No
Name
Address
Borrowed_By
Card_No
Acc_No
DOI
*
1
*
1
Sample data – library data
Acc_No Name Address Title Author YearofPub CardNo DOI
4 Doe Ray 4 sunshine lane Mindset Carol Dweck 2005 103 01/06/2013
8 Doe Ray 4 sunshine lane Learning Mindset Carol Dweck 2006 103 01/06/2013
2 Jacob John 7 Deer St Mindset Carol Dweck 2005 101 22/05/2013
1 John Smith 34 London Road Happy Days A.J. Roy 2009 100 23/07/2013
5 John Smith 34 London Road Learn to code Tom Close 2013 100 23/07/2013
6 John Smith 34 London Road Ruby on Rails M.H. Bedi 2010 100 23/07/2013
3 Mary jane 2 mill drive The art of writing M.B. Thomas 2001 102 15/08/2013
7 Mary jane 2 mill drive Learn to code Tom Close 2013 102 15/08/2013
9 Mary jane 2 mill drive ABC of photography K. Janice 2012 102 15/08/2013
Exercise: Managing a school
Create Students and Classes tables in a database for managing a school. • Each student can register for many classes and each class can
have many students. • Students must register for a class with their firstname, lastname
and age. They can select multiple classes within the same subject.
• Class has a class name and belongs to a subject and is taught in venue assigned to the class.
School data tables
Students
Student_ID
FirstName
LastName
Age
Classes
Class_id
className
Subject
StudentsClasses
Student_ID
Class_id
venue
*
1
*
1
Basic rules: • One table for one entity set • One column for one attribute
• One table for one relationship set • Columns are:
• Key attributes of all participating entity sets
• All descriptive attributes
1:1 Relationship
N:1 Relationship N:M Relationship
1:N Relationship
Types of Relationships
Example of 1:1 Relationship
1 1
Person
person_id
first_name
last_name
Spouse_contact
person_id
Spouse_name
Phone
Example of 1:N Relationship
* 1
Person
person_id
name
country_id
Country
country_id
country_name
capital
Example of N:M Relationship
*
1
Shop
shop_id
shop_name
location
Prices
product_id
shop_id
price
Product
product_id
product_name
category
1
*
A many-to-many relationship is one where two data tables or entities have multiple rows that are connected to one or more rows in the other table.
Process of Organising Your Data
What is Normalisation?
Why Normalise?
• Reduces redundancies and dependencies
• Saves space
• Reduces data anomalies
• Makes for easier maintenance
Normal Form
• Normal Forms (NF) are “standardised” rules
• Each builds off the previous form
• Higher the number the more normalised the data id (1NF, 2NF, 3NF, etc)
Example Data Parents student Name emergency contact student age classroom teacher school year grade level
John Smith John Smith Jr John Smith 10 C110 Ms Brown 2010 6
Mary Smith John Smith Jr John Smith 10 C110 Ms Brown 2010 6
John Smith John Smith Jr Mary Smith 10 C80 Mr Green 2010 6
Mary Smith John Smith Jr Mary Smith 10 C80 Mr Green 2010 6
John Smith April Smith John Smith 9 A25 Ms Baker 2009 5
Mary Smith April Smith John Smith 9 A25 Ms Baker 2009 5
John Smith April Smith Mary Smith 9 A99 Mr Close 2009 5
Mary Smith April Smith Mary Smith 9 A99 Mr Close 2009 5
Dave Harris Julie Harris Dave Harris 6 A10 Mr Jones 2010 3
Base Table We Will Start With
First Normal Form
• Remove “repeating” data
• A primary key can be defined
Primary key can uniquely identify any row in a table
Parents student Name emergency contact student age classroom teacher school year grade level
John Smith John Smith Jr John Smith 10 C110 Ms Brown 2010 6
Mary Smith John Smith Jr John Smith 10 C110 Ms Brown 2010 6
John Smith John Smith Jr Mary Smith 10 C80 Mr Green 2010 6
Mary Smith John Smith Jr Mary Smith 10 C80 Mr Green 2010 6
John Smith April Smith John Smith 9 A25 Ms Baker 2009 5
Mary Smith April Smith John Smith 9 A25 Ms Baker 2009 5
John Smith April Smith Mary Smith 9 A99 Mr Close 2009 5
Mary Smith April Smith Mary Smith 9 A99 Mr Close 2009 5
Dave Harris Julie Harris Dave Harris 6 A10 Mr Jones 2010 3
Second Normal Form
• Meet all requirements of 1NF
• Isolate repeated subsets of data
• Create the relationships
Normalised tables
* *
*
1
Normalised tables with relationships
1
1
*
1
Third Normal Form
• Meet all requirements of 2NF
• Pull out data that is not dependent on primary key
Exercise: Task Table
Task
task_id
Name
Due_date
description
project
completed
Project
project_id
name
Task
task_id
Name
Due_date
description
Project_id
completed
Allow Tasks to be assigned to a user (1:N relationship)
Task
task_id
Name
Due_date
description
Project_id
completed
User_id
User
user_id
name
*
1
Allow Tasks to be tagged with tags (N:M relationship)
Task
task_id
name
due_date
description
project_id
completed Tag
tag_id
name
TagTask
id
tag_id
task_id
*
1
* 1
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