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
- The integrity of data is the correctness
- 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
- E.g. A date field could be stored in a file as a text field but in another field as a date/time field
- Data Redundancy
- It refers to the unnecessary duplication of data
- No repeating attributes
- No piece of data should be unnecessarily repeated
- Data Integrity
- 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
- Has all nonkey attributes wholly dependent on the primary key
- 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
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