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28 Database Theory 28 Database Theory • • • • • Our world depends on databases Google basically IS a database (index) Facebook is build around databases So is e-Bay & Netflix and X-Box Live Travel agents, schools, doctors and hospitals all rely on them • You get the idea! 28 Database Theory • Databases are so useful because they’re so good at storing info • But they do it in a very structured way • That’s so the data can also be sorted • And so the data can also be searched • So overall, what makes databases so special is how they store, sort & search • Let’s look at some examples… 28 Database Theory Store Search Sort 28 Database Theory We’re actually going to look at databases into two stages. 28 Database Theory First, we’ll learn the basics of how databases work: A Database Tables B Types of Data C Relational Databases 28 Database Theory A Database Tables • So we’ve said what makes databases so special is how they store, sort & search • But how can they do that? • The secret is that they work in a very structured or organised way • This starts with the way that all data is stored in what are called tables… 28 Database Theory A Database Tables Here’s a database table for members of a club... Table Field name Record Field 28 Database Theory A Database Tables Here’s a database table for members of a club... Table Field name Record Field 28 Database Theory A Database Tables Here’s a database table for members of a club... Table Field name Record Primary Key Field 28 Database Theory A Database Tables Recap • • • • • Databases store info in a structured way Data in a database is stored in a table Each row is called a record Each column is called a field A special field called the primary key holds a unique identifier for each record 28 Database Theory are everywhere! ‘Unique identifiers’ 28 Database Theory A Database Tables Recap Table All this data is stored in a ?? This is a ?? Field This is a ?? Recordl This is the ?? Primary Key This is a ?? Field name 28 Database Theory A Database Tables Recap This table has ____ 4 records There are ____ 4 fields The primary key is called LessonID _______ 28 Database Theory A Database Tables B Types of Data C Relational Databases 28 Database Theory B Types of Data • A second secret to how databases work is the way they handle different types of data • Let’s look at an example… 28 Database Theory B Types of Data Can you spot the different types of data? We call these ‘data types’ Text Numbers Dates 28 Database Theory B Types of Data • Database tables can store different data types, such as... – Text – Numbers – Dates • When building a database, you say what type of data can go in each field • This can help prevent errors, such as checking that dates are DD/MM/YY 28 Database Theory A Database Tables B Types of Data C Relational Databases 28 Database Theory C Relational Databases • Simple databases with just one data table are called flat file databases. • But using only one table creates BIG problems for larger systems because lots of data can be duplicated and the database becomes very inefficient. • So advanced databases use multiple tables • They are called relational databases • Let’s look at why they are needed… 28 Database Theory C Relational Databases • Notice that several patients see Dr Brown • So her details are duplicated. • This is called redundant data because copies are inefficient & cause inconsistencies • Can you spot one? 28 Database Theory C Relational Databases PatientID Primary Key DocID Primary Key DocID Foreign Key 28 Database Theory C Relational Databases We show this relationship in a diagram Doctors Patients DocID Surname Department Mobile Patient ID Forename Surname DocID It’s a ‘One To Many’ relationship One doctor can treat many patients 28 Database Theory C Relational Databases Car Club • Again, data is duplicated or redundant • Redundant data causes inconsistencies • Can you spot one? 28 Database Theory C Relational Databases Car Club SupplierID 1 2 3 1 1 2 SupplierID 1 2 3 28 Database Theory C Relational Databases We show this relationship in a diagram Supplier Member SupplierID CarSupplier Address1 Town Postcode MemberID Forename Surname Car Reg Car Supplier It’s a ‘One To Many’ relationship One Car Supplier can supply many members 28 Database Theory C Relational Databases Dog Kennel Business • Again, data is duplicated or redundant • Redundant data causes inconsistencies • Can you spot any? 28 Database Theory C Relational Databases Dog Kennel Business VetID 3 1 2 2 3 2 VetID 1 2 3 Betta Vetz Top Vets West Vets 28 Database Theory C Relational Databases We show this relationship in a diagram Vet Pets VetID VetName Address1 Town Postcode PetID Name Owner Name Owner Mobile Vet ID It’s a ‘One To Many’ relationship One Car Supplier can supply many members 28 Database Theory