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1986, and there are now several versions of the language. Oracle conforms to the SQL-1999 core specification (often called SQL:99), which is the current minimum level for conforming to official SQL standards. Oracle extends the basic ANSI/ISO standard in several ways, making its own Oracle SQL far more powerful than the minimum acceptable SQL standards for the relational database industry. Relational database principles underlie SQL. You need only instruct the language what to do, not how to do it. In addition to working with traditional relational data, Oracle s new XML-centric extensions to its SQL language enable you to manage XML, full text, multimedia, and objects. Oracle Database 10g integrates XML query, storage, and update functionality in the database engine. No matter which tool you use to access the Oracle database, ultimately you ll be using Oracle SQL to perform your transactions. Your application program or the Oracle tool you use may allow you access to the database without your using SQL, but the tools and applications themselves have to use SQL to process your requests. SQL includes commands for data modeling, data definition, data access, data security, and data administration. SQL statements used by Oracle can be broadly divided into several groups based on whether they change the table data, the table structures, or some other session or instance characteristic. The SQL statement types are as follows: System control Session control Embedded SQL Data manipulation Transaction control Data definition The following sections examine each of these broad types of SQL statements in detail.

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You can use the system-control statement ALTER SYSTEM to alter the properties of a running database instance. For example, you can use ALTER SYSTEM to modify certain initialization parameters, such as the shared pool component of the system global area (SGA). At present, the ALTER SYSTEM command is the only system-control SQL statement in Oracle. Here s an example of the ALTER SYSTEM command: SQL> ALTER SYSTEM KILL SESSION '25,9192'; Session killed SQL>

Session-control statements dynamically alter the properties of an individual user s session. For example, if you intend to trace what your SQL session is doing in the database, you can use the ALTER SESSION SET SQL_TRACE=TRUE SQL statement to trace your session alone. The session-control statements also come in handy when you re changing several initialization parameters just for your session.

The rest of this chapter covers the potential as well as the limits and dangers of certain features related to mixed-code development.

PL/SQL (Oracle s procedural extension of the SQL language) doesn t support session-control statements.

Before proceeding we have to make sure the user s password has an expiration date set. If it doesn t, the else clause that follows will create a notification for the final report.

Common session-control statements include the ALTER SESSION and SET ROLE commands. Here s an example of the use of the ALTER SESSION statement, wherein the ALTER SESSION command is used to set the data format for the duration of the session: SQL> ALTER SESSION SET NLS_DATE_FORMAT = 'MM-DD-YYYY HH:MI:SS'; Session altered. SQL>

Embedded SQL statements incorporate DDL, DML, and transaction-control statements (such as OPEN, CLOSE, FETCH, and EXECUTE) used in a procedural language program, such as the statements used with the Oracle precompilers.

The data manipulation language (DML) statements are statements that either query (retrieve) or manipulate (change) data in a table. For the most part, DML statements modify the data in the schema objects. In most online transaction processing (OLTP) systems, the bulk of Oracle s work consists of accepting requests from users that contain DML statements and returning the results of those statements. You ll deal with four important DML statements most of the time: SELECT, INSERT, UPDATE, and DELETE. Note that in addition to these four common DML statements, there are others that facilitate the execution of the four basic DML statements. For example, the MERGE statement deals with conditional inserts and deletes, and the LOCK TABLE statement is used to prevent other transactions from modifying the same data while a transaction is still running.

SELECT statements are queries that retrieve data from a table or a set of tables (or views). Oracle provides set operators, such as union, minus, and intersection, that enable you to combine the results of several queries to get one final result set of data. You can use the ORDER BY command to sort the results provided by Oracle; otherwise, the results will not be in any particular order. When you need data from several tables, you need to join the tables in your SELECT statements. You can limit the result set when you join tables by providing a join condition. You can also use subqueries as part of the main or top query. A subquery in the WHERE clause of a SELECT statement is called a nested subquery. A subquery that is part of the FROM clause of a SELECT statement is called an inline view. The Appendix provides examples of subqueries, nested subqueries, and inline views.

This is the beginning of the core of the script. First we determine how many days have passed since the password was last changed. Then we evaluate this number to determine if the user should start receiving notifications that his account is about to be disabled.