CREATE THE DIFFERENCE Normalisation (special thanks to Janet Francis for this presentation)

CREATE THE DIFFERENCE Aim To demonstrate the meaning of normalisation To demonstrate how normalisation can be used to good effect 2

CREATE THE DIFFERENCE Normalisation A process which uses a set of rules for grouping data elements into logical entities (relations) If followed carefully, it will result in a robust database design Each stage in the process results in the production of a structure - a normal form. For most purposes, the first three stages (to 3 rd normal form – 3NF) are sufficient 3

CREATE THE DIFFERENCE Un-normalised data A list of fields needed for the system Scenario –All staff are released for two hours a week for staff development. –Employees work at their own pace in a lab. –A total of six attributes are recorded about each employee including their normal office location (building and room), the date they joined the course and how many hours it is planned for them to work on it.

CREATE THE DIFFERENCE Un-normalised data Course ID Course Name Employee ID Name Building Room ID Date Joined Course Allocated Hours In this example, Course ID, Employee ID and Room ID are known to be unique.

CREATE THE DIFFERENCE Problems There is no record of the employee until they have joined a course. Lots of duplicate employee data is created once employees start to join courses.

CREATE THE DIFFERENCE First Normal Form (1NF) An entity is in 1NF if it has an identifying key and there are no repeating attributes or repeating groups of attributes To get to 1NF we must remove all repeating groups

CREATE THE DIFFERENCE Remember what we started with Course details and Employee details are repeating groups Course ID Course Name Employee ID Name Building Room ID Date Joined Course Allocated Hours

CREATE THE DIFFERENCE We need to: Take one of the unique identifiers eg. “Course ID” (we could have used Room ID or Employee ID) For each of the other attributes, check if they have a one to one relationship with “Course ID” If so, keep them, if not move them into a new entity. For the new entity, a unique identifier is required and is formed as a composite by using “Course ID” combined with another unique identifier. “Employee ID” is chosen for this example though it could have been Room ID.

CREATE THE DIFFERENCE Our Example COURSEEMP_ON_COURSE Course ID Course Name Course ID*, Employee ID Name Building Room ID Date Joined Course Allocated Hours NB: Course ID is part of the composite Primary Key of the new entity “ EMP_ON_COURSE”. It is also the Foreign Key providing a relationship with COURSE

CREATE THE DIFFERENCE Problems Still problem with employee details Not so much duplicate data – course details are now only entered once per course.

CREATE THE DIFFERENCE Second Normal Form (2NF) An entity is in 2NF if it is in 1NF and has no attributes which require only part of the key to identify them uniquely To get to 2NF we remove part key dependencies All data items must be dependant on the whole of the composite primary key

CREATE THE DIFFERENCE Not all groups are in 2NF COURSE is already in 2NF EMP_ON_COURSE is not because AttributeDepends On Name Block Room ID Date Joined Course Hours Employee ID Employee ID + Course ID

CREATE THE DIFFERENCE So we.. Take out details that are linked only to “Employee ID” into a separate entity. If in any doubt, ask a question such as ‘Are these fields affected when an Emloyee joins a course’ AttributeDepends On Name Building Room ID Employee ID

CREATE THE DIFFERENCE ….end up with three entities COURSEEMP_ON_COURSEEMPLOYEE Course ID Course Name Course ID*, Employee ID* Date Joined Course Allocated Hours Employee ID Name Building Room ID The two parts of the composite Primary key in EMP_ON_COURSE are Foreign keys in the linked tables

CREATE THE DIFFERENCE Problems No problem with courses No problem with Employees But –Building and Room Number are related in that a room is in a particular building. If one is updated the other will be affected. –If the building names change, then with the current structure the whole of the employee records currently stored will have to be updated

CREATE THE DIFFERENCE Third Normal Form (3NF) An entity is in 3NF if it is in 2NF and no non-key attribute depends on another non-key attribute. To get to 3NF we must remove attributes that depend on other non-key attributes i.e. resolve the Room and Building problem

CREATE THE DIFFERENCE We need to: Decide on the direction of the dependency between the attributes For example –If, given a value for A, there is only one possible value for B, then A determines B B is dependant on A –So for rooms at Staffordshire University, the room number is unique – we know for example that K342 is in the Octagon Building and C312 is in the Beacon Building. –If you know the room, you can find out the building - the same is not true vice-versa because if you know the Building you cannot determine the room.

CREATE THE DIFFERENCE And the solution is.. Leave Room ID in the original entity as a foreign key, but remove Building into a separate entity with Room ID as the Primary Key. EMPLOYEEROOM Employee ID Name Room ID* Room ID Building

CREATE THE DIFFERENCE Entity Relationship Modelling Course Emp_On_CourseEmployee Room This is not perfect – why – at least 3 reasons

CREATE THE DIFFERENCE To Normalise to 3NF 1. Remove all repeating data elements and ensure that everything is dependant on the Primary Key 2. Ensure data items are dependant on the whole of the composite primary key 3. Remove to new entities all fields dependant on non-key fields This process is sometimes referred to as The key, the whole key and nothing but the key!!

CREATE THE DIFFERENCE Ways to represent Normalisation To make it easier to write down –# represents a numeric field –Primary keys are underlined –Foreign keys* are in Italics with an asterisk The entities we created would be represented as: EMPLOYEE (#Employee ID,Name,#Room ID*) ROOM (#Room No, Building) COURSE (#Course ID, Course Name) EMP_ON_COURSE (#Course ID*, #Employee ID*, Date Joined Course, allocated Hours