4.3.6 - Database Systems - Relational Databases

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  • Created by: Annie
  • Created on: 05-05-13 19:23
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  • Relational Databases
    • "A large collection of data items stored in two or more linked tables so that data can be accessed in many different ways and by a range of applications"
    • Advantages
      • Data Integrity
        • The integrity  of data is the correctness
          • The extent to which it truthfully represents the original information
        • One problem of maintaining integrity arises when updating occurs
          • In a flat file database, if one record was left unchanged the data would be no longer wholly correct
          • In a relational database, the data would only have to be changed in one table and all the references will be automatically updated
      • Data Consistency
        • E.g. A date field could be stored in a file as a text field but in another field as a date/time field
          • The data would be incompatible
        • When data is held in more than one file, it should be stored in a consistent way
        • In a relational database because the attributes of any one entity are contained within one file, there is no risk of the same attribute being stored in a different format in a different file
      • Data Redundancy
        • It refers to the unnecessary duplication of data
        • No repeating attributes
        • No piece of data should be unnecessarily repeated
    • Data Independence
      • The data and the programs used to access it are independent or separate
      • Refers to the ability to add or change the data in a database within affecting applications linked to it
        • Not the fields or database structure
        • e.g. adding new customers to receive an email
      • This means that new applications can more easily be developed to link to existing databases
        • e.g. mobile phone application to link a company's stock database to check on stock levels before a sales rep makes a sale
      • Advantage
        • If you want to program a new app, don't need to create a new contacts database
    • Data Normalisation
      • Good database design
      • Normalisation should ensure that there will be no update, insert or deletion anomalies
      • 1st Normalisation
        • "Remove repeating fields or groups of data"
        • For the defined primary key, identify fields where one or many records may exist
      • 2nd Normal Form
        • Has all nonkey attributes wholly dependent on the primary key
          • Remove composite keys adding a primary key as needed
        • Identify many to many relationships and resolve
          • A many to many relation doesn't enable one record to link specifically to other records
      • 3rd Normal Form
        • No functional dependency between non-key items
        • This means that "Are any of the non-key fields dependent upon other non-key fields?"
          • If so, they must be separated into furhter tables with primary and foreign keys used to link the data items appropriately

Comments

Mr A Gibson

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A decent way of organising the main points relating to relational databases. Use this to test yourself on definitions and explanations. Get to grips with normalisation - use this to work through an example (see this resource on page 5-7 as a basic example).

Chizismygame

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Cheers GIBBO you're a fuc@ing ledge geez

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