Unit 4 Normalisationand Relational Database Management Systems

Computerised Databases A database is a organised collection of data items and links between them structured in such a way that it allows it to be accessed by a number of different application programs. i.e. related tables of data which are linked together and many programs can use the data in the tables Use a database when you need to store lots of data

Members No ForenameSurnameAddress 1Address 2Tel NoVideo No 122EvansDaniel7 High StAberaeron LeesAnna2 Bridge St Lampeter EvansDaniel7 High StAberaeron Title Certificate Category Cost per day Date loaned Length of loan Date due Total cost Crash12 Comedy £3.0012/09/051 day13/09/05£3.00 Emma12Drama£3.0012/09/051 day13/09/05£3.00 War of the Worlds PGSci FI£3.5914/09/052 days16/09/05£3.50 Duplication in Flatfile

Linked data tables in a relational database Primary Key Foreign Key Primary Key Loans ID Primary Key

Normalisation

Terminology: Primary Key A primary key is a unique value which allows each record to be identified CustomerIDFirstNameLastName 1BrianSmith 2HarryAdams 3JoeJones 4HarrySmith  FirstName or LastName cannot be primary keys as they contain duplicate and un-unique data. CustomerID uniquely identifies a row and is therefore suitable

Sometimes there is no single field appropriate as a primary key. In these circumstances, it is possible to select two fields which, when taken together create a unique value: OrderNoItemNoEmployeeNoCustomerNoItemNameQuantity Nut Bolt Washer Bolt Washer4 There are no unique fields, so the Primary Key is best suited by OrderNo and ItemNo taken together

Foreign Key A Foreign key is a primary key of another table used to link the tables

Second Normal Form (2NF) To be in 2NF a table must: –Be in 1NF (obviously) –Have all non key fields fully functionally dependant on the primary key In English: –A non key field is one that is not part of the primary key –It means that you need to use the primary key to determine the value of the other fields in the table –If you can find the value of other fields without using the primary key, you should remove that field from the table and place it in a separate table

Third Normal Form (3NF) To be in 3NF a table must: –Fields which do not form part of the primary key must always be solely dependent on the primary key and not on anything else, such as another non key field

Third Normal Form (Cont.) Remove City from the table and create a new cities table Remove Salary from the table and create a job type table Staff EmployeeID* LastName FirstName CityID TypeID Cities CityID* City JobTypes TypeID* Salary

WardID # No of Beds Type of Ward DoctorID * PatientID#Name Illness Gender Address WardID * DoctorID# Name Specialism ContactNo DoctorTable (DoctorID#, Name, Specialism, Contact No, WardID,) WardTable (WardID#, No of Beds,Type of Ward, DoctorID) PatientTable (PatientID#, Name,Adress, Illness, Gender, DoctorID, WardID), # Primary Key * Foreign Key

Advantages of using databases (Advantages of normalisation) 1. Avoids data duplication -data stored once -linked by keyfields -all data available via relational links in keyfields 2. Controlled redundancy - Minimises data duplication 3. Ensures consistency of data - of data to all users 4. Data independence - data stored separately from programs so can add new fields because data is independent of the applications which use it 5. Increased security

What makes databases secure ? Hierarchy of passwords User access rights and privileges Data is stored separately from programs so different programs cannot overwrite data.

Disadvantages Complex to set up and maintain; needs team of programmers to maintain it. Database software is large complex expensive and requires powerful computers. All applications which access the data will be affected if database fails. As the DBMS is the only access to operational data a system failure can have serious consequences