Database Systems

There a many different database systems that have been used throughout the years, all of which have their own strengths and weaknesses. A data model is an assortment of rules for the description of the constraints, data types and relationships of a database.
There are five different database systems customarily used, two of which – network and hierarchical - are predecessors of the relational model. The other two commonly used models are the object-oriented and object-relational models.
Relational Database

The relational model is the best known and most implemented database model at present. Invented in 1970 by E.F. Codd at IBM, it describes a database as a collection of tables, or relations, which contain all data. From these tables, data can be accessed and amended without having to reorganise the tables.

The typical program interface for a relational database is the structured query language, or SQL. SQL statements are used both for interactive queries for information from a relational database and for gathering data for reports.

In addition to being relatively easy to create and access, a relational database has the important advantage of being easy to extend. After the original database creation, a new data category can be added without requiring that all existing applications be modified.

A relational database is a set of tables containing data fitted into predefined categories. Each table (which is sometimes called a relation) contains one or more data categories in columns. Each row contains a unique instance of data for the categories defined by the columns. For example, a typical business order entry database would include a table that described a customer with columns for name, address, phone number, and so forth. Another table would describe an order: product, customer, date, sales price, and so forth. A user of the database could obtain a view of the database that fitted the user's needs. For example, a branch office manager might like...