Introduction to SQL

Introduction to SQL
In this section we outline the objectives of SQL, provide a short history of the language,
and discuss why the language is so important to database applications.
Objectives of SQL
Ideally, a database language should allow a user to:
n create the database and relation structures;
n perform basic data management tasks, such as the insertion, modification, and deletion
of data from the relations;
n perform both simple and complex queries.
A database language must perform these tasks with minimal user effort, and its command
structure and syntax must be relatively easy to learn. Finally, the language must be
portable, that is, it must conform to some recognized standard so that we can use the same
command structure and syntax when we move from one DBMS to another. SQL is
intended to satisfy these requirements.
SQL is an example of a transform-oriented language, or a language designed to use
relations to transform inputs into required outputs. As a language, the ISO SQL standard
has two major components:
a Data Definition Language (DDL) for defining the database structure and controlling
access to the data;
n a Data Manipulation Language (DML) for retrieving and updating data.
Until SQL:1999, SQL contained only these definitional and manipulative commands; it
did not contain flow of control commands, such as IF . . . THEN . . . ELSE, GO TO, or
DO . . . WHILE. These had to be implemented using a programming or job-control language,
or interactively by the decisions of the user. Owing to this lack of computational
completeness, SQL can be used in two ways. The first way is to use SQL interactively
by entering the statements at a terminal. The second way is to embed SQL statements in
a procedural language, as we discuss in Appendix E. We also discuss SQL:1999 and
SQL:2003 in Chapter 28.
SQL is a relatively easy language to learn:
n It is a non-procedural language: you specify what information you require, rather than
how to get it. In other words, SQL does not require you to specify the access methods
to the data.
n Like most modern languages, SQL is essentially free-format, which means that parts of
statements do not have to be typed at particular locations on the screen.
n The command structure consists of standard English words such as CREATE TABLE,
INSERT, SELECT. For example:
– CREATE TABLE Staff (staffNo VARCHAR(5), lName VARCHAR(15),
salary DECIMAL(7,2));
– INSERT INTO Staff VALUES (‘SG16’, ‘Brown’, 8300);
– SELECT staffNo, lName, salary
FROM Staff
WHERE salary > 10000;
n SQL can be used by a range of users including Database Administrators (DBA), management
personnel, application developers, and many other types of end-user.
An international standard now exists for the SQL language making it both the formal and
de facto standard language for defining and manipulating relational databases (ISO, 1992,
1999a).