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Transcript
44220: Database Design & Implementation Modelling the ‘Real’ World Ian Perry Room: C49 Ext.: 7287 E-mail: [email protected] http://itsy.co.uk/ac/0506/sem2/44220_DDI/ Last Lecture ended with this … This Lecture: Ian Perry Explores the complex idea of ‘Data Modelling’. 44220: Database Design & Implementation: Modelling the ‘Real’ World Slide 2 What is this? Ian Perry 44220: Database Design & Implementation: Modelling the ‘Real’ World Slide 3 And this? Any of the family of aquatic birds, especially those having short legs, webbed feet, and a broad blunt bill. Collins Concise English Dictionary Ian Perry 44220: Database Design & Implementation: Modelling the ‘Real’ World Slide 4 And this? dTw k 1 TE TW k 2V k 3F TA TW dt Ian Perry 44220: Database Design & Implementation: Modelling the ‘Real’ World Slide 5 And this? Duck • What do they have in common? • They are ALL, depending upon your information requirements, perfectly adequate models of a Duck. Ian Perry 44220: Database Design & Implementation: Modelling the ‘Real’ World Slide 6 So, what is a Model? Always remember that; They are: Models ARE NOT the Real thing. the appearance of reality. an analogue of the real world. a simplified representation of reality. the abstraction of meaning (the semantics). We build models for a purpose, so; Ian Perry being clear as to the purpose of a model is the key to success. 44220: Database Design & Implementation: Modelling the ‘Real’ World Slide 7 Every-day Models Model Duck: Model Aeroplane: Purpose; to show shape, colour, size, etc. Purpose; to show general structure, identification of parts, flight characteristics, etc. Data Model: Ian Perry Purpose; the representation of objects of interest to an enterprise, allowing data to be structured (i.e. given meaning) and manipulated (for specific purposes). 44220: Database Design & Implementation: Modelling the ‘Real’ World Slide 8 The Data Abstraction Process Requires us to focus on the critical aspects of the real world’s richness. All models require decisions about: no model is complete! what to include & what to exclude? These decisions are represent someone’s view of a particular reality, i.e.; Ian Perry what is important, and MUST be included. what is not important, and can be left out. 44220: Database Design & Implementation: Modelling the ‘Real’ World Slide 9 Data Models in the Plural The complexity of the database design process means that we must use a hierarchy of data models: Conceptual Data Model Logical Data Model Physical Data Model Successful database design requires; Ian Perry an orderly, and rigorous, progression through this ‘Data Modelling Stack’. 44220: Database Design & Implementation: Modelling the ‘Real’ World Slide 10 The ‘Data Modelling Stack’ Conceptual Overview of things Logical Physical Ian Perry that are perceived to be of ‘interest’ in the ‘real’ world. Data elements & the relationships between those elements in a tabular form. Actual data held in a database & the means to manipulate that data. Model of the Business System. (ER Model) Model of Data Storage Theory (Db Schema) Physical Implementation (RDBMS) 44220: Database Design & Implementation: Modelling the ‘Real’ World Slide 11 Conceptual Data Model Initial ‘view’ of the Objects of interest; An integrated ‘view’ of the whole; which is compromise free AND Software & Hardware independent. Conceptual Data Modelling, is all about; their properties, relationships, semantics. ‘capturing’ WHAT real-world Objects MUST be included, i.e. to suit a particular purpose. We will build Conceptual Models using: Ian Perry Entity Relationship (ER) Diagrams 44220: Database Design & Implementation: Modelling the ‘Real’ World Slide 12 Logical Data Model Logical Data Modelling, is all about; We will build these using: ‘deciding’ HOW to best represent the properties of the Objects of interest, and the relationships between them. Database Schema A Database Schema: Ian Perry defines the single, integrated, data collection that is the whole database. within the constraints of a chosen/imposed theoretical framework. 44220: Database Design & Implementation: Modelling the ‘Real’ World Slide 13 Theories of Logical Modelling Several to choose from, e.g.: Each Logical Modelling Theory comes with its own: Hierarchical, Relational, Object-based, etc. Data Definition Language (DDL) Data Manipulation Language (DDM) Data Query Language (DQL) Software availability tends to influence the Logical Modelling Theory chosen here: Ian Perry As we will be using Microsoft Access (a Relational DataBase Management System), our logical choice must be Relational. 44220: Database Design & Implementation: Modelling the ‘Real’ World Slide 14 Physical Data Model Physical Data Modelling, is all about; This process may be influenced by; ‘mapping’ a Logical Data Model onto a specific Physical Storage System. both Software specific & Hardware specific considerations. i.e. one may have to compromise the Logical Data Model; Ian Perry in order to make it ‘fit’ the Software (in our case Microsoft Access) that will be used build, maintain & manipulate the Physical Data Model. 44220: Database Design & Implementation: Modelling the ‘Real’ World Slide 15 Database Design & Development Analyse Develop a logical data model (constrained by a particular database theory); from the conceptual data model. Implement a conceptual data model; from the above analysis. Develop a business situation; in order to ‘discover’ the purpose. a physical data model (constrained by software availability); based on the logical data model. Manipulate Ian Perry i.e. test the physical data model; to prove it ‘suits’ the purpose discovered by the original analysis. 44220: Database Design & Implementation: Modelling the ‘Real’ World Slide 16 DDI Assessment Method Assignment 1 (50%) [01 Mar, 2006] Develop; an ‘appropriate’ Conceptual Data Model for a Case Study organisation. Develop; a ‘robust’ Logical Data Model based on the Conceptual Data Model. Assignment 2 (50%) [28 Apr, 2006] Ian Perry Implement; a ‘accurate’ Physical Data Model based on the Logical Data Model. Manipulate; the Physical Data Model to ‘prove it can’ Answer Specific Questions from the Case Study. 44220: Database Design & Implementation: Modelling the ‘Real’ World Slide 17 This Week’s Workshop In this Workshop we will begin to explore some of the decisions we must make as we attempt to construct a model of ‘real’ world ‘things’. Working alone, or in a small team, consider the following half dozen ‘things’ that exist in the ‘real’ world. For each of the above; list as many as you can imagine (try to be as creative & inventive as you can) of the ways that it might be modelled. For each way of modelling the above ‘things’; University, Computer, Bank, Shoe Shop, Doctor’s Surgery, Sports Activity. indicate who might regard this form of model as a meaningful and useful abstraction that captures its essential aspects. DO NOT arrive at the workshop un-prepared! Ian Perry 44220: Database Design & Implementation: Modelling the ‘Real’ World Slide 18