Databases

Key Definitions

§ A file is a collection of sets of similar data called records

§ A table is another name for a file

§ A tuple is another name for a record

§ An item is an item of data within a recorded

§ A field is the area where items are stored

§ An attribute is another name for a field

§ A database is a series of related files, called tables

§ The primary or key field is a unique field in a record

§ A foreign key is the field which contains a link to another field

§ A secondary key is a field in a table that can be used to access the data in different ways

Flat and Relational databases

§ A flat file database is a single dimensioned collection of data

§ There are several limitations to flat file databases:

o Separation and isolation of data

o Duplication of data

o Data dependence

o Incompatibility of files

o Fixed queries and the proliferation of programs

§ A Relational database links different fields between tables, which increases the efficiency and manages resources better through a more dynamic approach.

Arranging data

§ A normal form is the name given to how data in a database is arranged

§ First normal form (1NF) is where each table has no repeating groups

§ Second normal form (2NF) is where the values of the attributes are all dependant on the primary key

§ Third normal form (3NF) is where no attributes are predictable because of one of the other attributes

Analysing and implementing database design

§ The primary key indicating a unique tuple by which a record can be identified, must be shown by putting it in brackets and underlining it.

§ A many-to-one or one-to-many relationship is where there is one record linking to many other records in another table.

§ A many-to-many relationship is where many records link to many more records.

§ A link table is the table which is used to store many-to-many relationships

§ A link entity is another name for a link table

Database management systems

§ A database management system (DBMS) is a piece of software that allows a database to be managed and controlled.

§ Data description language (DDL) is included in the DBMS. It is used to define tables and the data types and data structures for files, and any restrictions.

§ The schema is the design that is created by the DDL

§ Different users with different privileges will be able to see different parts of the database, called the subschema.

§ Data manipulation language (DML) is also included in the DBMS. It will allow users to manipulate the data. Depending on their rights, different users will be able to do different things with the data.

§ A data dictionary contains all file descriptions on the structure and type of data, it is stored within the DBMS.

§ With the use of various tools, the DBMS allows data to be presented in a variety of different ways:

o Internal level is the view of the entire database as it is stored in the system

o Conceptual level gives a single usable view of all the data in the database

o External level is where data is arranged according to the user requirements and rights

§ SQL is a simple language that allows users to set up their own queries on a database