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Databases - Coggle Diagram

- - - - A change is performed or not performed
      - Half-finished changes must not be saved
    - - Any change must retain the overall state of the database
      - E.g. money out of one account must be balanced by money in to another account
    - - A transaction must not be able to be interrupted by a different transaction
      - It is locked until the change is committed
    - - Changes must be preserved, even in the event of failure
  - - - Forces records to be consistent and accurate
    - - Ensures that if the primary key in one table is changed, then the foreign keys in other tables are also updated
      - Forces the rule that a primary key cannot be duplicated
- - - - All records are stored in one place
      - Easy to set up using a number of standard office applications
      - Easy to understand
      - Simple sorting of records can be carried out
      - Record can be viewed or extracted on the basis of simple criteria
      - Everyday things like business contacts, customer lists and so on can be stored and used
    - - Potential duplication / non-unique records
      - Harder to update - could have to be updated over many records
      - Inherently inefficient - each record has to have the same fields, whether they are used or not
      - Harder to change data format (e.g. adding a dash in the middle of telephone numbers across tens of thousands of records)
      - Poor at complex queries
      - Poor at limiting access
  - - - No multiple record changes needed
      - More efficient storage
      - Simple to delete or modify details
      - Complex queries can be carried out using SQL
      - Better security - certain tables can be made confidential
      - Cater for future requirements - it is simple to add records that are not yet needed but may be needed in the future
- - - - Needs to be resolved
      - Example
        
        Each Order has many Products
        
        Each Product has many Orders
        
        Create a table called Order_has_Product
        
        Each record consists of an OrderID and a ProductID
        
        The primary key is a composite of the two