Chapter 5

Chapter 5

Database Terminology
1. Table - a data structure made up of rows and columns that contains data about the items.
2. Record - a single row within a table.
3. Field - an individual data item within a record.
4. Entity - a table.
5. Attribute - fields within an entity.
Keys
1. Primary key - a field, in the table, that allows each record to be uniquely identified.
  1. Simple - made up of a single field only.
  2. Composite - combines more than one field to make a unique value.
2. Secondary key - a field that is identified as being suitable for indexing the data.
3. Foreign key - a field in one table that is linked to a primary key in another table.
Relationships
1. One to one - any instance of the entity X can be associated with only one instance of entity Y.
2. Many to many - single instance of an entity can be associated with many instances of another entity.
Referential Integrity
1. Referential integrity – for every foreign key there must be a related primary key value.
2. Makes sure that it is impossible to enter a reference to a link which does not exist.
Data Dictionary
1. Contains metadata (data about data).
2. Table name - name of table, must be unique.
3. Field name - each field uniquely identified.
4. Field data type - data type allocated to each field.
5. Field length - no. of characters allocated for field contents.
6. Field default value - value appears on creation of new record.
7. Field validation - any validation applied to field.
8. Table security - who has access to edit table.
9. Keys - primary and foreign keys.
10. Indexes - any field which is indexed.
11. Relationships - relationships between tables identified.
Queries
1. Simple - a query where there is only one parameter.
2. Complex - a query with more than one parameter. Makes use of AND, OR and NOT to join parameters.
3. Static - parameter is hard coded into the query and cannot be changed by the end user.
4. Dynamic - request for the parameter given to the end user at run time, usually by a dialogue box.
Normalisation
1. UNF
2. 1NF
  1. All data is atomic.
  2. Record has a primary key.
  3. Each field name is unique.
  4. No repeating fields.
3. 2NF
  1. Table is in 1NF.
  2. All non-key fields be found out by using the entire primary key.
4. 3NF
  1. Table is in 2NF.
  2. All non-key items are fully dependent on the primary key.
5. Evaluation
  1. Advantages
    1. No redundant data.
    2. Easier to maintain.
    3. Efiicient structure.
    4. Flexible, can be altered without major redesign.
  2. Disadvanatges
    1. Reduced performance.
    2. Can't store historical data.
    3. More storage space required.

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	Created by smita089 about 11 years ago