INTRODUCTION TO SQL

SQL (Structured Query Language) is a special purpose programming language designed for managing data held in a relational database management system (RDBMS).

Originally based upon relational algebra and tuple relational calculus, SQL consists of a data definition language and a data manipulation language. The scope of SQL includes data insert, query, update and delete schema creation and modification, and data access control. Although SQL is often described as, and to a great extent is, a declarative language (4GL), it also includes procedural elements.

SQL was one of the first commercial languages for Edgar F. Codd's relational model, as described in his influential 1970 paper "A Relational Model of Data for Large Shared Data Banks".Despite not entirely adhering to the relational model as described by Codd, it became the most widely used database language.

SQL became a standard of the American National Standards Institute (ANSI) in 1986, and of the International Organization for Standards (ISO) in 1987. Since then, the standard has been enhanced several times with added features. Despite these standards, code is not completely portable among different database systems, which can lead to vendor lock-in. The different makers do not perfectly adhere to the standard, for instance by adding extensions, and the standard itself is sometimes ambiguous.

Language elements

The SQL language is subdivided into several language elements, including:

·         Clauses, which are constituent components of statements and queries. (In some cases, these are optional.)

·         Expressions, which can produce either scalar values, or tables consisting of columns and rows of data.

·         Predicates, which specify conditions that can be evaluated to SQL three-valued logic (3VL) (true/false/unknown) or Boolean truth values and which are used to limit the effects of statements and queries, or to change program flow.

·         Queries, which retrieve the data based on specific criteria. This is an important element of SQL.

·         control transactions, program flow, connections, sessions, or diagnostics.

·         SQL statements also include the semicolon (";") statement terminator. Though not required on every platform, it is defined as a standard part of the SQL grammar.

OPERATORS

Operator	Description	Example
=	Equal to	Author = 'Alcott'
<> or !=	Not equal to	Dept <> 'Sales'
>	Greater than	Hire_Date > '2012-01-31'
<	Less than	Bonus < 50000.00
>=	Greater than or equal	Dependents >= 2
<=	Less than or equal	Rate <= 0.05
BETWEEN	Between an inclusive range	Cost BETWEEN 100.00 AND 500.00
LIKE	Match a character pattern	First_Name LIKE 'Will%'
IN	Equal to one of multiple possible values	DeptCode IN (101, 103, 209)
IS or IS NOT	Compare to null (missing data)	Address IS NOT NULL

                                                                 Table 1.3 Operators in SQL