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44220: Database Design & Implementation Avoiding Database Anomalies Ian Perry Room: C49 Tel Ext.: 7287

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Presentation on theme: "44220: Database Design & Implementation Avoiding Database Anomalies Ian Perry Room: C49 Tel Ext.: 7287"— Presentation transcript:

1 44220: Database Design & Implementation Avoiding Database Anomalies Ian Perry Room: C49 Tel Ext.:

2 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies Avoiding Database Anomalies This lecture concentrates upon building a ‘robust’ Logical Data Model. i.e.: Transforming a Conceptual Data Model into a set of Relations. Checking these Relations for any Anomalies. Documenting them as a Database Schema. Most Database books have a section describing a mathematically-based technique called Normalisation: I will show you a much easier way of achieving the same result, i.e. a robust database design.

3 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies What is an Anomaly? Anything we try to do with a database that leads to unexpected and/or unpredictable results. Three types of Anomaly to guard against: insert delete update Need to check your database design carefully: the only good database is an anomaly free database.

4 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies Insert Anomaly When we want to enter a value into a data cell but the attempt is prevented, as another value is not known.  e.g. We have built a new Room (e.g. B123), but it has not yet been timetabled for any courses or members of staff.

5 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies Delete Anomaly When a value we want to delete also means we will delete values we wish to keep.  e.g. CoNo 351 has ended, but Room C320 will be used elsewhere.

6 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies Update Anomaly When we want to change a single data item value, but must update multiple entries  e.g. Room H940 has been improved, it is now of RSize = 500.

7 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies A Conceptual Model Consider the following ‘simple’ conceptual data model: Staff(Staff-ID, Name, Address, ScalePoint, RateOfPay, DOB,...) Student(Enrol-No, Name, Address, OLevelPoints,...) Course(CourseCode, Name, Duration,...) Staff Course Student 1MMM

8 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies The ‘Translation’ Process Entities become Relations Attributes become Attributes(?) Key Attribute(s) become Primary Key(s) Relationships are represented by additional Foreign Key Attributes: for those Relations that are at the ‘M’ end of each 1:M Relationship.

9 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies The ‘Staff’ & ‘Student’ Relations Staff(Staff-ID, Name, Address, ScalePoint, RateOfPay, DOB,...) becomes: Staff Student(Enrol-No, Name, Address, OLevelPoints,...) becomes: Student NB. Foreign Key Tutor references Staff.Staff-ID

10 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies The ‘Staff’ & ‘Course’ Relations Course(CourseCode, Name, Duration,...) becomes: Course NB. Can’t add a Foreign Key; as BOTH Relations have a ‘M’ end:  I warned you about leaving M:M relationships in your Conceptual Data Model.  Must create an ‘artificial’ linking Relation. Staff

11 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies ‘ Staff’, ‘Course’ & ‘Team’ Relations NB. In the ‘artificial’ Relation (i.e. Team): The Primary Key is a composite of CourseCode & Staff-ID Foreign Key CourseCode references Course.CourseCode Foreign Key Staff-ID references Staff.Staff-ID Team Course Staff

12 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies 4 Relations from 3 Entities? BUT - are they anomaly free? Student Team Course Staff

13 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies Check Relations for Anomalies! every Tuple unique? no hidden meaning from location? data cells atomic? for Relations with single-attribute keys: every Attribute depends upon the Primary Key? for Relations with composite keys: every Attribute depends upon all of the Composite Key?

14 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies What if the checks fail? If any Relation fails ‘checks’: especially those checking dependency. we MUST split that Relation into multiple Relations: until they pass the tests. but MUST remember to leave behind a Foreign Key: to ‘point’ forwards to the Primary Key of the ‘new’ split-off Relation.

15 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies Are they Anomaly Free? Student Team Course Staff NOT this one! as RateOfPay does NOT depend upon Staff-ID

16 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies Fixing this ‘Problem’ The Attribute ‘RateOfPay’ depends upon ‘ScalePoint’ NOT ‘Staff-ID’. So, we need to split this Relation: NB. Foreign Key ScalePoint references Pay.ScalePoint Staff Pay Staff

17 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies 5 Relations from 3 Entities Student Team Course Pay Staff an ‘artificial’ Relation - to ‘solve’ a M:M ‘problem’ a ‘split-off’ Relation - to ‘solve’ a Dependency ‘problem’

18 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies Don’t change Conceptual Model Remember, we can chose from one of a range of Database Theories with which to build our Logical Data Model: Hierarchical Relational Object Each of these Database Theories may require different compromises (i.e. at the Logical Modelling stage); from the ‘pure’ meaning captured by your Conceptual Model.

19 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies Document Relations as a Database Schema A Database Schema: defines all Relations, lists all Attributes (with their Domains), and identifies all Primary & Foreign Keys. We should have ‘captured’ the Business situation (assumptions and constraints) in the Conceptual Data Model, e.g: a College only delivers 10 Courses. a Hospital only has 12 Wards. These assumptions and constraints need to be expressed as the Domains of the Database Schema.

20 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies Logical Schema 1 - Domains Schema College Domains StudentIdentifiers = ; StaffIdentifiers = ; PersonNames = TextString (15 Characters); Addresses = TextString (25 Characters); CourseIdentifiers = ; CourseNames = Comp, IS, Law, Mkt,...; OLevelPoints = ; ScalePoints = ; PayRates = £14,005, £14,789, £15,407,...; StaffBirthDates = Date (dd/mm/yyyy), >21 Years before Today;

21 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies Logical Schema 2 - Relations Relation Student Enrol-No: StudentIdentifiers; Name: PersonNames; Address: Addresses; OLevelPoints: OLevelPoints; Tutor: StaffIdentifiers; Primary Key: Enrol-No Foreign Key Tutor references Staff.Staff-ID

22 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies Logical Schema 3 - Relations Relation Staff Staff-ID: StaffIdentifiers; Name: PersonNames; Address: Addresses; ScalePoint: ScalePoints; DOB: StaffBirthDates; Primary Key: Staff-ID Foreign Key ScalePoint references Pay.ScalePoint

23 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies Logical Schema... Relation Course CourseCode: CourseIdentifiers; Name: CourseNames; … etc. Continue to define each of the Relations in a similar manner. All Relations MUST have a Primary Key. Any Relation at the M-end of a 1:M Relationship MUST have a Foreign Key. Make sure that you define ALL of the Relations, including: ‘artificial’ ones (e.g. Team) ‘split-off’ ones (e.g. Pay)

24 Ian PerrySlide : Database Design & Implementation: Avoiding Database Anomalies This Week’s Workshop In this Workshops session we will; 1. test a logical data model; to ensure that it is anomaly free (i.e. robust), 2. practice documenting a Database Schema; based on a small conceptual model (as represented by an ER Diagram). 1.Examine a table of data: Explain the ‘potential’ for insert, delete & update anomalies in a table of data. Define what a ‘better’ set of tables (Relations?) to store the data look like? 2.Examine an ER Diagram: Identify suitable Attributes for each Relation; as a minimum those that will act as the Primary & Foreign Keys. Document as a Database Schema; starting with the Relations first, then coming back to document suitable Domains.


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