Normalisation Unit 6: Databases

Just to recap  What is an Entity  What is an Attribute?

Just to recap  So, for example:  An entity in a video store database would be ‘Customer’  What would the Attributes be?

Attributes  First Name  Surname  Address  Telephone Number  Age  Gender 

Introduction  This week we will be going on to standard database notation and Normalisation  In your assignment your plan will need to be in both tabular format and in standardised database notation

Lesson Objectives  By the end of this lesson, you should be able to: Design a database in standard database notation Define the three forms of normalisation

Standard database notation  Using the video store as an example:

Standard database notation  In a video store database, the tables can be described as follows CUSTOMER (CustomerID, Firstname, Surname, Address, TelephoneNumber, , Age, Gender) VIDEO (VideoID, VideoTitle, Rating, FilmGenre, DateReleased, LengthOfFilm, RentalPrice) VIDEORENTALS (VideoRentalID, CustomerID, VideoID, DateRented, DateReturned)

Primary Keys  The underlined field types are known as Primary Keys This is a field that is used to uniquely define a particular record or line in a table. Since text fields e.g. surname can be repeated, primary keys are nearly always numeric fields.  They can include: Membership number, product number, employee number, catalogue number, account number.

Primary and Foreign Key Customer Table Customer ID First Name Surname Address Telephone number Age Gender Video Table Video ID Video title Rating Film Genre Date Released Length of film Rental price Video Rentals Table Video_Rental ID Customer ID Video ID Date Rented Date Returned

Your Task  For the two databases you have designed: Draw the tables as shown in the previous slide Identify possible Primary Keys

Foreign Keys  This is a field in one table which is also the primary key of another table.  Foreign keys are used to establish a relationship between the main table and other subsidiary tables.  Looking at the video tables, what would be the foreign keys?

Foreign Keys Customer Table Customer ID First Name Surname Address Telephone number Age Gender Video Table Video ID Video title Rating Film Genre Date Released Length of film Rental price Video Rentals Table Video_Rent al ID Customer ID Video ID Date Rented Date Returned

Your Task  On the tables you have finished for your databases, identify the foreign keys

Normalisation  The process of refining the structure of a database to minimise redundancy and improve integrity is called normalisation.  When a database has been normalised it is said to be in normal form.  There are three normal forms: First, second and third

First Normal Form  A database is in first normal form if there are no repeated fields.  That means that there must only be one field for each item of data you want to store.  A library database, for example, is not in first normal form because it has the field’s book1, book2, book3, book4, book5, and book6, which all store the same information, i.e. the name of a book.

First Normal Form  First normal form not only facilitates searching, but is also more space efficient as there wouldn't be 5 empty fields being stored if borrowers only had one book.

Second Normal Form  A database is said to be in second normal form if it: is already in first normal form has no fields that aren't dependent on the whole of the key  That is to say that all fields are dependent on the whole of the key (where there is a compound key).

Second Normal Form  For example, a merits database: Name Tutor group Teacher Subject Date Reason Head of year Tutor  There are no repeated fields, so it's in first normal form. If you made name the key field, however, you could see there would be fields that aren't dependent on the key.

Second Normal Form  Subject, for example, is related to the teacher, not to the student. This database, therefore, is not in second normal form.  Even if you were to make a compound key to include the date and the teacher giving the merit, it still wouldn't be in second normal before because subject wouldn't be dependent on the whole of the key.

Third Normal Form  A database is in third normal form if it: is already in second normal form it has no non-key dependencies  By non-key dependencies, we mean that there are no fields that are dependent on other fields that are not part of the key.

Third Normal Form  For example: In the version of merits database previously, where ‘Name’ was the key, the ‘subject’ was related to the ‘teacher’, and not to the student - that is a non-key dependency.

Real Life  But forget all that! It still sounds a bit complicated to me, and I've been developing relational databases (including professionally) for more than 4 years.  For your assignment, don't try to start at first normal form, and then go to second normal form, and then to third.  Once you've got the hang of it, you'll knock out databases in third normal form without even thinking about it. The key thing is to think about what your entities (or tables) are going to be - if you pick the right ones it'll normalise itself.

The Normalisation Oath  Well, don't forget it all, however, because you may be asked about normalisation in an exam. Here's a little tip to remember the three stages - we call it the Normalisation Oath:  Each attribute is dependent on the key, the whole key, and nothing but the key!  So, to get to third normal form, your non- repeating fields (first normal form) need to be dependent on the whole of the key (second normal form), and nothing other than the key (third normal form). It works for me!

So,  By choosing the correct field names and tables, your database should normalise itself  Normalisation improves searches and queries