Relational Databases

A relational database is a database that is perceived by its users as a collection of relations or tables. The tables are the logical structure in a relational system, not the physical structure. So tables represent an abstraction of the way the data is physically stored. The storage level details are hidden from the user. The entire information content of the database is represented as explicit data values and all data values are atomic. So at every row and column position in every table there is always exactly one data value, never a group of several values.

The formal theory underlying relational systems is called the relational model. The relational model is concerned with the logical matters only, not physical matters. Relational model addresses three aspects of data, data structure, data integrity and data manipulation. The relational model is a theory and database systems may not support every detail of the theory. However, there are certain rules that the database must obey to conform to the relational model.

A Normalized database means that a database's tables have been set up so that no information is duplicated between tables and key fields offer a reference point between related tables. Normalization is to get rid of redundant information, to make updates easier, and to save storage space. There are three different normalization levels, or forms. If a relation is said to be in first normal form if and only if each attribute of the relation is atomic. In other words, each column must contain only a single value and each row must contain the same columns. If a relation is said to be in second normal form if and only if it satisfy the conditions for first normal form and each non-key attribute in the relation must be functionally dependent upon the primary key. If a relation is said to be in third normal form if and only if it satisfy the conditions for second normal form and all attributes that are not dependent upon the primary key must be eliminated.

SQL Structured Query Language) is used to communicate with a database. According to ANSI (American National Standards Institute), it is the standard language for relational database management systems. SQL statements are used to perform tasks such as update data on a database, or retrieve data from a database.

Today, just about all large-enterprise information systems rely on relational database management systems like Oracle, Sybase and DB2.

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