Revised by Ivor Perry Sept Introduction to Data Modelling

Revised by Ivor Perry Sept Data Modeling in IS Development Information System (IS) –“A system that processes data to produce information” When analysing the existing system –View of current data in the system (existing data view) –Current processes that operate on the data and/or produce data (process view) –Making sure these tally (event/transaction view)

Revised by Ivor Perry Sept –A view of proposed data in the system (proposed data view) –Required processes to operate on data and/or produce data (process view) to satisfy some user requirement –Making sure these tally (event/transaction view) Note No mention of a computer system or computerisation! When designing a new system

Revised by Ivor Perry Sept Entity-Relationship Modelling Top-down approach to data modelling Start by identifying the entities and their associations/relationships Draw up an ER Diagram (ERD) –using appropriate notation Applying rules, translate ERD into a set of well-normalised tables The creative bit is the identification of appropriate entities and their relationships The rest is learning how to adhere to and apply appropriate rules of ER modelling correctly

Revised by Ivor Perry Sept ER Modelling - How it works Entity –thing –independent existence –uniquely identifiable –something you need to keep a list of! E.g. employee, order, machine, property Nouns Defined in terms of their properties

Revised by Ivor Perry Sept Attributes Attribute –property of an entity e.g. –an employee’s attributes might be: national insurance number, salary, date of birth –an order’s attributes might be: date of order, order number, customer ID –a machine’s attributes might be: model number, capacity, description –a property’s attributes might be: property address, number of bedrooms, price

Revised by Ivor Perry Sept Relationships Relationship –association of some kind between entities Verbs e.g. employee and machine –relationship might be works on An employee works on exactly one machine order and stock –relationship might be relates to An order relates to many stock items

Revised by Ivor Perry Sept Identifying Entities and Attributes How to tell what is –an appropriate entity? –an appropriate attribute? Experience –practice makes better Reading hints in scenario –if scenario based - especially in exams! Acting on information from fact finding investigations Never expect to get it right the first time in the real world –(or the second!) –E-R modelling is an iterative process –It is a CREATIVE process

Revised by Ivor Perry Sept Reading the hints in the scenario or acting on information from fact finding investigations Boundary of the problem –e.g, client wants a computerised leisure centre booking system a computerised video rental system you to design a booking system for a village hall Entities actually highlighted explicitly –“record details of bookings, videos and…” Attributes actually highlighted even if entities aren’t –“the secretary keeps a record of the order number, date of order and what items of stock are held together with their respective quantity”

Revised by Ivor Perry Sept Nouns and verbs –sometimes provide a hint as to entities and their relationships can see them as Enterprise Rules e.g. “a customer can make several orders, but an order relates to just one customer” Look for Enterprise Rules either at attribute level or at entity level “a part description can be the same for several part numbers, although part numbers are unique” “A part may be used in several machine repairs, and a machine repair may involve at least one part”

Revised by Ivor Perry Sept Entity Type vs Entity Occurrence Project vs “P1, Machine Deployment System” Employee vs “E1, MR JG Browning” Machine vs “Lathe, #10110” Entity-Relationship Modelling == Entity(Type)-Relationship Modelling But: We are interested in Entity(Occurrence)-Relationship(Occurrence) Diagrams to illustrate concepts (more later)

Revised by Ivor Perry Sept Notations Many notations are used for Entity-Relationship modelling. The one I have used here (the Howe notation) is particularly good for beginners. Seeing examples of more than one notation is useful as so many exist. The concepts are the same whatever the notation.

Revised by Ivor Perry Sept ER Diagram An ER Diagram is a pictorial representation of Entity (type) level enterprise rules within the boundary of what exists or is required Basic notation of ER Diagram: –NB - the colours are not part of the syntax = Entity = Relationship EmployeeMachine works on Always hard to name the relationship

Revised by Ivor Perry Sept Degree of a Relationship 1:1 Relationship (one to one) EmployeeMachine Works on 11 Enterprise Rules Represented: “An employee works on at most one machine” “A machine is worked on by at most one employee”

Revised by Ivor Perry Sept Employee“Works on”Machine E1M1 E2M2 E3M3 E4M4 E5 Entity Relationship Occurrence Diagram 1:1

Revised by Ivor Perry Sept :M Relationship (one to many) Naming convention from 1 to M here Often we name relationships from M to 1 EmployeeMachine Works on M1 Enterprise Rules Represented: “ An employee may work on one or many machines” “A machine is worked on by at most one employee”

Revised by Ivor Perry Sept Entity Relationship Occurrence Diagram 1:M Employee“Works on”Machine E1M1 E2M2 E3M3 E4M4 E5

Revised by Ivor Perry Sept M:N Relationship (many to many) EmployeeMachine Works on NM Enterprise Rules Represented: “An employee may work on one or many machines” “A machine is worked on by one or many employees” Equally you could use M:M instead of M:N

Revised by Ivor Perry Sept Entity Relationship Occurrence Diagram M:M Employee“Works on”Machine E1M1 E2M2 E3M3 E4M4 E5

Revised by Ivor Perry Sept Complex Relationships Typically M:N:P for 3 entity relationships (M:M:M) –M:N:P:R for 4 entity relationships (very rare) or even more!!!! CustomerPart supplies PM Supplier N Enterprise Rules Represented: –“A supplier may supply one or more parts to one or more customers” –“A customer may be supplied with one on more parts from one or more suppliers” –“A part may be supplied by one or more suppliers to one or more customers”

Revised by Ivor Perry Sept End of Lecture