Databases

Databases

1. What is a database?

   • A database is a way of organising information stored on a computer using up little space which is accessible to a user in various ways.
   • RDBMS – relational database management system

2. Database termoniligy

   • Keys
     • Primary Key
       • A primary key is a field in a table which is unique and enables you to identify every record in the field.
     • Foreign Key
       • A foreign key is used to link tables together and create a relationship. It is a field in once table that is linked to the primary key in another table.
     • Compound key
       • A compound primary key combines more than field to make a unique value
     • Secondary Key
       • A secondary key is made on a field that you would like to be indexed for faster searches. A table can have more than one secondary key
   • Why use?
     • Vast amounts of information can be stored.
     • Queries can be run to search for specific records or groups of records.
     • Reports can be produced from the data stored or queries run
     • Mail merge can be run through exporting data from the database into a word processing doccument
     • Validation can be used to reduce errors
   • Table
     • collection of records
   • Record
     • A record is all of the data or information about one person or one thing.
   • Field
     • Attribute about a record(s) columns in a table

3. Database Notation

   • The table name should be in capital letters
   • The attributes should be placed in brackets following the table name
   • The attributes should be seperated by a comma
   • The primary key should be underlined
   • The foreign key should be overlined
   • If an attribute contains two or more words, they should be joined together and the start of each word should have a capital letter

4. Entities and Attributes

   • Entity
     • An entity is a person , place, thing or concept about which data can be collected
   • Attribute
     • an attribute describes the facts, details or characteristics of an entity
   • Entity Relationship Diagrams
     • One to One
     • One to Many
     • Many to Many

5. Normiliseation

   • The purpose of normalisation is to reduce redundancy and inconsistency and make it easier to use and maintain.
   • 0NF
     • • Data might be repeated • Data might be no automatic • Table might not have a primary key • Repeating field name
   • 1NF
     • • There are no columns with repeated or similar data • Each data item cannot be broken down any further • Each row is unique • Each field has a unique name
   • 2NF
     • • Table must be in 1NF • Non-key attributes must depend on every part of the primary key.
   • 3NF
     • • Table must be in 2NF already • There are no non-key attributes that depend on another key attributes

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