Introduction to Relational Databases

Relational databases are probably the most common type of database used for general-purpose tasks. In a relational database, information is grouped according to its type, generally in tables (see below). For example, in a database designed to hold fleet information you may include a table of employees and a table of vehicles.
- In addition to separating information according to its data structure, a relational database allows relationships to be created. A relationship defines a possible link between data types; the actual linkage of data is dependent upon the information held.
- Relational databases use the concept of normalization. Normalization is a design technique that minimizes the duplication of information. It also reduces the risk of errors. By using relationships, the duplication required can be lessened or eliminated completely.
A Relational model is the basis for any relational database management system (RDBMS). A relational model has mainly three components:
- A collection of objects or relations.
- Operators that act on the objects or relations.
- Data integrity methods.

Elements of a Relational Database Schema :
There are several key elements to a relational database. Each of these forms a part of the database's schema. The schema is the logical data model that determines the information that may be stored in the database and how it is to be arranged. To design a database we need three things:
- Table : A table is one of the most important ingredient to design the database. It is also known as a relation, and is a two dimensional structure used to hold related information. A database consists of one or more tables.
- Rows : A table contains rows. Rows are collection of instance of one thing.
- Columns : A table contains the columns. Columns contains all the information of a single type. Each column in a table is a category of information referred to as a field.
- Indexes : One of the greatest benefits of holding information in a database is the ability to quickly retrieve it. When querying a database, it is possible to apply criteria to ask for a specific set of rows.
- Keys : A primary key is a single column, or group of several columns (compound key), that can be used to uniquely identify rows in a table. Each table in a database may have a single primary key. Once defined, no two rows in the table may contain matching data in the primary key columns. Foreign keys are used when defining relationships between tables. A foreign key is a single column, or group of columns, in a table that reference the primary key in another table. This creates a link between the two tables.
- Constraints : Constraints are rules that are applied to the information in a database. These are usually used to enforce business rules upon the tabular data.
- Views : Views provide the useful concept of virtual tables. A view gathers specific information from one or more sources and presents it in the format of a single table. The information may be filtered within the view to remove unnecessary information.
- Stored Procedures : A stored procedure is a predefined set of statements that can be executed when required. Stored procedures provide the main means of creating programs within SQL Server databases.

Domain and Integrity Constraints :
* Domain Constraints
o limit the range of domain values of an attribute
o specify uniqueness and `nullness' of an attribute
o specify a default value for an attribute when no value is provided.
* Entity Integrity
o every tuple is uniquely identified by a unique non-null attribute, the primary key.
* Referential Integrity
o rows in different tables are correctly related by valid key values (`foreign' keys refer to primary keys).