Introduction to Oracle SQL

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.

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 Database, 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 the PL/SQL extension to Oracle SQL is similar to PSM.

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 Database 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 preceding tasks in one consistent language.

Lexical Conventions

The following lexical conventions for issuing SQL statements apply specifically to the Oracle Database 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 Database evaluates the following two statements in the same manner:

SELECT last_name,salary*12,MONTHS_BETWEEN(hire_date, SYSDATE)
FROM employees
WHERE department_id = 30
ORDER BY last_name;

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

Case is insignificant in reserved words, keywords, identifiers and parameters. However, case is significant in text literals and quoted names. Please refer to "Text Literals" for a syntax description of text literals.