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Introduction To SQL


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Structured Query Language (SQL) is the set of statements with which all programs and users access data in an Oracle database. Application programs and Oracle tools often allow users access to the database without using SQL directly, but these applications in turn must use SQL when executing the user's request. This chapter provides background information on SQL as used by most database systems.

This chapter contains these topics:

* History of SQL

* SQL Standards

* Embedded SQL

* Lexical Conventions

History of SQL
Dr. E. F. Codd published the paper, "A Relational Model of Data for Large Shared Data Banks", in June 1970 in the Association of Computer Machinery (ACM) journal, Communications of the ACM. Codd's model is now accepted as the definitive model for relational database management systems (RDBMS). The language, Structured English Query Language ("SEQUEL") was developed by IBM Corporation, Inc., to use Codd's model. SEQUEL later became SQL (still pronounced "sequel"). In 1979, Relational Software, Inc. (now Oracle Corporation) introduced the first commercially available implementation of SQL. Today, SQL is accepted as the standard RDBMS language.

SQL Standards
Oracle Corporation strives to comply with industry-accepted standards and participates actively in SQL standards committees. Industry-accepted committees are the American National Standards Institute (ANSI) and the International Standards Organization (ISO), which is affiliated with the International Electrotechnical Commission (IEC). Both ANSI and the ISO/IEC have accepted SQL as the standard language for relational databases. When a new SQL standard is simultaneously published by these organizations, the names of the standards conform to conventions used by the organization, but the standards are technically identical.

The latest SQL standard was adopted in July 1999 and is often called SQL:99. The formal names of this standard are:

* ANSI X3.135-1999, "Database Language SQL", Parts 1 ("Framework"), 2 ("Foundation"), and 5 ("Bindings")

* ISO/IEC 9075:1999, "Database Language SQL", Parts 1 ("Framework"), 2 ("Foundation"), and 5 ("Bindings")

How SQL Works

The strengths of SQL provide benefits for all types of users, including application programmers, database administrators, managers, and end users. Technically speaking, SQL is a data sublanguage. The purpose of SQL is to provide an interface to a relational database such as Oracle, and all SQL statements are instructions to the database. In this SQL differs from general-purpose programming languages like C and BASIC. Among the features of SQL are the following:

* It processes sets of data as groups rather than as individual units.

* It provides automatic navigation to the data.

* It uses statements that are complex and powerful individually, and that therefore stand alone. Flow-control statements were not part of SQL originally, but they are found in the recently accepted optional part of SQL, ISO/IEC 9075-5: 1996. Flow-control statements are commonly known as "persistent stored modules" (PSM), and Oracle's PL/SQL extension to SQL is similar to PSM.

Essentially, SQL lets you work with data at the logical level. You need to be concerned with the implementation details only when you want to manipulate the data. For example, to retrieve a set of rows from a table, you define a condition used to filter the rows. All rows satisfying the condition are retrieved in a single step and can be passed as a unit to the user, to another SQL statement, or to an application. You need not deal with the rows one by one, nor do you have to worry about how they are physically stored or retrieved. All SQL statements use the optimizer, a part of Oracle that determines the most efficient means of accessing the specified data. Oracle also provides techniques that you can use to make the optimizer perform its job better.

SQL provides statements for a variety of tasks, including:

* Querying data

* Inserting, updating, and deleting rows in a table

* Creating, replacing, altering, and dropping objects

* Controlling access to the database and its objects

* Guaranteeing database consistency and integrity

SQL unifies all of the above tasks in one consistent language.

Common Language for All Relational Databases

All major relational database management systems support SQL, so you can transfer all skills you have gained with SQL from one database to another. In addition, all programs written in SQL are portable. They can often be moved from one database to another with very little modification.

Embedded SQL
Embedded SQL refers to the use of standard SQL statements embedded within a procedural programming language. The embedded SQL statements are documented in the Oracle precompiler books.

Embedded SQL is a collection of these statements:

* All SQL commands, such as SELECT and INSERT, available with SQL with interactive tools

* Dynamic SQL execution commands, such as PREPARE and OPEN, which integrate the standard SQL statements with a procedural programming language

Embedded SQL also includes extensions to some standard SQL statements. Embedded SQL is supported by the Oracle precompilers. The Oracle precompilers interpret embedded SQL statements and translate them into statements that can be understood by procedural language compilers.

Each of these Oracle precompilers translates embedded SQL programs into a different procedural language:

* Pro*C/C++ precompiler

* Pro*COBOL precompiler

* SQL*Module for ADA

Lexical Conventions
The following lexical conventions for issuing SQL statements apply specifically to Oracle's implementation of SQL, but are generally acceptable in other SQL implementations.

When you issue a SQL statement, you can include one or more tabs, carriage returns, spaces, or comments anywhere a space occurs within the definition of the statement. Thus, Oracle evaluates the following two statements in the same manner:

SELECT last_name,salary*12,MONTHS_BETWEEN(hire_date, SYSDATE)
FROM employees;

SELECT last_name,
salary * 12,
MONTHS_BETWEEN( hire_date, SYSDATE )
FROM employees;

Case is insignificant in reserved words, keywords, identifiers and parameters. However, case is significant in text literals and quoted names.

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