SQL

Structured Query Language. The SQL language has several parts:

DDL – Data definition language: The SQL DDL provides commands for defining

Relation schemas, defining relations, and modifying relation schema.

View definition: The SQL DDL includes commands for defining views.

DML-Data Manipulation Language. The SQL DML includes a query language based on both the relational algebra and the tuple relational calculus.

Transaction control: To specify begin and end of transactions, SQL commands is used.

Integrity: The SQL DDL includes commands for specifying integrity constraints that  the stored in the database must satisfy. Updates that violate integrity constraints are disallowed.

Authorization: The SQL DDL includes commands for specifying access rights to relations and views.

Basic structure:

The Basic structure of an SQL expression consists of three clauses: select, from and where.

• Select – select clause corresponds to the projection operation of the relational algebra. It is used to list the attributes desired in the result of a query.
• From – This clause corresponds to the Cartesian product operation of the relation algebra.
• Where – The where clause corresponds to the selection predicate of the relational algebra.

SQL Query has the form

Select A1, A2…..An

From r1, r2,……..rm

Where p

A- Attribute, r- relation, p-predicate.

DDL: SQL DDL allows specification of not only a set of relations but also information about each relation including:

• Schema for each relation.
• Integrity constraints.
• Domain of values associated with each attribute.
• Security and authorization information for each relation.
• Physical storage structure of each relation on disk.

Domain Types in SQL:

It supports a variety of built-in domain types:

int: An integer. The full form integer is equivalent.

Smallint: A small integer

Char(n) : A fixed length character string with user specified length n.

Varchar(n): A variable length character string with user specified maximum length n.

Float(n): A floating point number,with precision of atleast n digits.

Date: A calendar date containing a year, month and day of the month.

Time: time of day, in hours, minutes and seconds.