The Relational Database Model:

Databases were designed based on the network and the hierarchical models. In 1970, Edgar F. Codd proposed the relational model. This model gained prominence over the years and began to dominate the data processing applications scenario in business organizations.

The Relational Database Model, also known as the Relational Database Management System (RDBMS) is based on the concept of tables or relations. In this model, the data is organized in the form of tables. A table consists of records and each record contains columns, also known as fields. The fields contain attributes like employee ID, employee name, age, etc. while a row (also known as a tuple) identifies the details pertaining to a single instance like the details of a particular employee.

In relational databases, each row is distinctive and each column has a unique name. Each row has a key that identifies the row with the relation. Data records stored in tables can be accessed using a primary key. The primary key is a field in a record that is used to distinguish a record from other records in the table. For example, employee number will be the primary key in the employee table.

Relational databases allow the designating of certain fields as keys to facilitate specific field value search, i.e., when two different tables constitute the same field values, a single operation can be performed by matching the field values to select the related records in the two tables. At times, the fields will be referred to by the same name in both the tables. For instance, a table containing the personal details of the employees will have a field for employee ID. A table containing the official details of the employees will also have an employee ID field. This employee ID field can be used as a medium for relating the two tables. This facility offered by relational databases can also be extended to relate multiple tables.

Relational databases also have keys called foreign keys. These keys are primary keys of one table which can be included in another table for relating or joining the data. For example, the social security numbers of the father and mother can be used to represent their child. In this example, the social security number of the father and/or the mother acts as a primary key in a table that represents them while it acts as a foreign key in the table which represents their child.

Relational database management systems like DB2, Microsoft SQL Server, Oracle, Informix, Sybase SQL Server, Ingres, SQLite, Access, etc. are based on the relational model.

Structured Query Language:

The database interrogation capability of a DBMS allows end users with no programming knowledge to use the system. The system's query language feature lets users obtain immediate responses to ad hoc data requests.

Query language is a specialized language used to request information from a database. Query languages can be procedural or non-procedural languages. A procedural language is one for which the user provides instructions to the system for performing a sequence of operations on the database. The system takes these instructions and computes the desired information. In non-procedural languages, the user requests the desired information but does not give a procedure for obtaining it. SQL, MDX, DMX, Datalog, XQuery, XPointer, XPath, SMARTS, SPARQL, etc. are examples of some query languages.

The Structured Query Language (SQL, pronounced as Sequel) is a popular query language. It is a non-procedural query language. SQL has become a de facto standard for query languages because of its wide usage and recognition in the industry. It is a standard language used for relational databases. It includes both data definition and data manipulation operations. A SQL query consists of a command keyword followed by additional information. By combining keywords, users can create queries that extract just the information they are looking for. The basic command consists of the following three subcommands:

SELECT: lists the data items to be retrieved.
FROM: lists the tables from which the data will be retrieved.
WHERE: specifies the conditions and parameters which will limit the search.
Let us assume that the table ‘salesdetails' has the following data items:
salesperson, city, state, sales
Let us suppose that the marketing manager of a company wants to know the sales details in Rajasthan state (coded as RJ), for all salespersons. The commands that the manager can use are:
SELECT salesperson, city, state, sales
FROM salesdetails
WHERE state = ‘RJ'

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