Skip Headers
Oracle® Database PL/SQL Language Reference
11g Release 2 (11.2)

Part Number E17126-03
Go to Documentation Home
Home
Go to Book List
Book List
Go to Table of Contents
Contents
Go to Index
Index
Go to Master Index
Master Index
Go to Feedback page
Contact Us

Go to previous page
Previous
Go to next page
Next
View PDF

1 Overview of PL/SQL

PL/SQL, the Oracle procedural extension of SQL, is a portable, high-performance transaction-processing language. This chapter explains its advantages and briefly describes its main features and its architecture.

Topics:

Advantages of PL/SQL

PL/SQL has these advantages:

Tight Integration with SQL

PL/SQL is tightly integrated with SQL, the most widely used database manipulation language. For example:

  • PL/SQL lets you use all SQL data manipulation, cursor control, and transaction control statements, and all SQL functions, operators, and pseudocolumns.

  • PL/SQL fully supports SQL data types.

    You need not convert between PL/SQL and SQL data types. For example, if your PL/SQL program retrieves a value from a column of the SQL type VARCHAR2, it can store that value in a PL/SQL variable of the type VARCHAR2.

    You can give a PL/SQL data item the data type of a column or row of a database table without explicitly specifying that data type (see "%TYPE Attribute" and "%ROWTYPE Attribute").

  • PL/SQL lets you run a SQL query and process the rows of the result set one at a time (see "Processing a Query Result Set One Row at a Time").

PL/SQL supports both static and dynamic SQL. Static SQL is SQL whose full text is known at compilation time. Dynamic SQL is SQL whose full text is not known until run time. Dynamic SQL enables you to make your applications more flexible and versatile. For more information, see Chapter 6, "PL/SQL Static SQL" and Chapter 7, "PL/SQL Dynamic SQL".

High Performance

PL/SQL lets you send a block of statements to the database, significantly reducing traffic between the application and the database.

Bind Arguments

When you embed a SQL INSERT, UPDATE, DELETE, or SELECT statement directly in your PL/SQL code, the PL/SQL compiler turns the variables in the WHERE and VALUES clauses into bind arguments (for details, see "Resolution of Names in Static SQL Statements"). Oracle Database can reuse these SQL statements each time the same code runs, which improves performance.

PL/SQL does not create bind arguments automatically when you use dynamic SQL, but you can use them with dynamic SQL by specifying them explicitly (for details, see "EXECUTE IMMEDIATE Statement").

Subprograms

PL/SQL subprograms are stored in executable form, which can be invoked repeatedly. Because stored subprograms run in the database server, a single invocation over the network can start a large job. This division of work reduces network traffic and improves response times. Stored subprograms are cached and shared among users, which lowers memory requirements and invocation overhead. For more information about subprograms, see "Subprograms".

Optimizer

The PL/SQL compiler has an optimizer that can rearrange code for better performance. For more information about the optimizer, see "PL/SQL Optimizer".

High Productivity

PL/SQL lets you write compact code for manipulating data. Just as a scripting language like PERL can read, transform, and write data in files, PL/SQL can query, transform, and update data in a database.

PL/SQL has many features that save designing and debugging time, and it is the same in all environments. If you learn to use PL/SQL with one Oracle tool, you can transfer your knowledge to other Oracle tools. For example, you can create a PL/SQL block in SQL Developer and then use it in an Oracle Forms trigger. For an overview of PL/SQL features, see "Main Features of PL/SQL".

Portability

You can run PL/SQL applications on any operating system and platform where Oracle Database runs.

Scalability

PL/SQL stored subprograms increase scalability by centralizing application processing on the database server. The shared memory facilities of the shared server enable Oracle Database to support thousands of concurrent users on a single node. For more information about subprograms, see "Subprograms".

For further scalability, you can use Oracle Connection Manager to multiplex network connections. For information about Oracle Connection Manager, see Oracle Database Net Services Reference.

Manageability

PL/SQL stored subprograms increase manageability because you can maintain only one copy of a subprogram, on the database server, rather than one copy on each client system. Any number of applications can use the subprograms, and you can change the subprograms without affecting the applications that invoke them. For more information about subprograms, see "Subprograms".

Support for Object-Oriented Programming

PL/SQL supports object-oriented programming with "Abstract Data Types".

Support for Developing Web Applications

PL/SQL lets you create applications that generate web pages directly from the database, allowing you to make your database available on the Web and make back-office data accessible on the intranet.

The program flow of a PL/SQL Web application is similar to that in a CGI PERL script. Developers often use CGI scripts to produce web pages dynamically, but such scripts are often not optimal for accessing the database. Delivering Web content with PL/SQL stored subprograms provides the power and flexibility of database processing. For example, you can use DML statements, dynamic SQL, and cursors. You also eliminate the process overhead of forking a new CGI process to handle each HTTP request.

You can implement a Web browser-based application entirely in PL/SQL with PL/SQL Gateway and the PL/SQL Web Toolkit.

PL/SQL Gateway enables a Web browser to invoke a PL/SQL stored subprogram through an HTTP listener. mod_plsql, one implementation of the PL/SQL Gateway, is a plug-in of Oracle HTTP Server and enables Web browsers to invoke PL/SQL stored subprograms.

PL/SQL Web Toolkit is a set of PL/SQL packages that provides a generic interface to use stored subprograms invoked by mod_plsql at run time. For information about packages, see "Packages".

See Also:

Oracle Database Advanced Application Developer's Guide for information about developing PL/SQL Web applications

Support for Developing Server Pages

PL/SQL Server Pages (PSPs) let you develop web pages with dynamic content. PSPs are an alternative to coding a stored subprogram that writes the HTML code for a web page one line at a time.

Special tags let you embed PL/SQL scripts into HTML source code. The scripts run when Web clients, such as browsers, request the pages. A script can accept parameters, query or update the database, and then display a customized page showing the results.

During development, PSPs can act like templates, with a static part for page layout and a dynamic part for content. You can design the layouts using your favorite HTML authoring tools, leaving placeholders for the dynamic content. Then, you can write the PL/SQL scripts that generate the content. When finished, you simply load the resulting PSP files into the database as stored subprograms.

See Also:

Oracle Database Advanced Application Developer's Guide for information about developing PSPs

Main Features of PL/SQL

PL/SQL combines the data-manipulating power of SQL with the processing power of procedural languages.

When you can solve a problem with SQL, you can issue SQL statements from your PL/SQL program, without learning new APIs.

Like other procedural programming languages, PL/SQL lets you declare constants and variables, control program flow, define subprograms, and trap run-time errors.

You can break complex problems into easily understandable subprograms, which you can reuse in multiple applications.

Topics:

Error Handling

PL/SQL makes it easy to detect and handle errors. When an error occurs, PL/SQL raises an exception. Normal execution stops and control transfers to the exception-handling part of the PL/SQL block. You do not have to check every operation to ensure that it succeeded, as in a C program. For more information, see Chapter 11, "PL/SQL Error Handling".

Blocks

The basic unit of a PL/SQL source program is the block, which groups related declarations and statements.

A PL/SQL block is defined by the keywords DECLARE, BEGIN, EXCEPTION, and END. These keywords divide the block into a declarative part, an executable part, and an exception-handling part. Only the executable part is required. A block can have a label.

Example 1-1 shows the basic structure of a PL/SQL block. For syntax details, see "Block".

Example 1-1 PL/SQL Block Structure

<< label >> (optional)
DECLARE    -- Declarative part (optional)
  -- Declarations of local types, variables, & subprograms

BEGIN      -- Executable part (required)
  -- Statements (which can use items declared in declarative part)

[EXCEPTION -- Exception-handling part (optional)
  -- Exception handlers for exceptions (errors) raised in executable part]
END;

Declarations are local to the block and cease to exist when the block completes execution, helping to avoid cluttered namespaces for variables and subprograms.

Blocks can be nested: Because a block is an executable statement, it can appear in another block wherever an executable statement is allowed.

You can submit a block to an interactive tool (such as SQL*Plus or Enterprise Manager) or embed it in an Oracle Precompiler or OCI program. The interactive tool or program runs the block one time. The block is not stored in the database, and for that reason, it is called an anonymous block (even if it has a label).

An anonymous block is compiled each time it is loaded into memory, and its compilation has three stages:

  1. Syntax checking: PL/SQL syntax is checked, and a parse tree is generated.

  2. Semantic checking: Type checking and further processing on the parse tree.

  3. Code generation

Note:

An anonymous block is a SQL statement.

Variables and Constants

PL/SQL lets you declare variables and constants, and then use them wherever you can use an expression. As the program runs, the values of variables can change, but the values of constants cannot. For more information, see "Declarations" and "Assigning Values to Variables".

Subprograms

A PL/SQL subprogram is a named PL/SQL block that can be invoked repeatedly. If the subprogram has parameters, their values can differ for each invocation. PL/SQL has two types of subprograms, procedures and functions. A function returns a result. For more information about PL/SQL subprograms, see Chapter 8, "PL/SQL Subprograms."

PL/SQL also lets you invoke external programs written in other languages. For more information, see "External Subprograms".

Packages

A package is a schema object that groups logically related PL/SQL types, variables, constants, subprograms, cursors, and exceptions. A package is compiled and stored in the database, where many applications can share its contents. You can think of a package as an application.

You can write your own packages—for details, see Chapter 10, "PL/SQL Packages." You can also use the many product-specific packages that Oracle Database supplies. For information about these, see Oracle Database PL/SQL Packages and Types Reference.

Triggers

A trigger is a named PL/SQL unit that is stored in the database and run in response to an event that occurs in the database. You can specify the event, whether the trigger fires before or after the event, and whether the trigger runs for each event or for each row affected by the event. For example, you can create a trigger that runs every time an INSERT statement affects the EMPLOYEES table.

For more information about triggers, see Chapter 9, "PL/SQL Triggers."

Input and Output

Most PL/SQL input and output (I/O) is done with SQL statements that store data in database tables or query those tables. For information about SQL statements, see Oracle Database SQL Language Reference.

All other PL/SQL I/O is done with PL/SQL packages that Oracle Database supplies, which Table 1-1 summarizes.

Table 1-1 PL/SQL Packages that Process Input and Output

Package Description More Information

DBMS_OUTPUT

Lets PL/SQL blocks, subprograms, packages, and triggers display output. Especially useful for displaying PL/SQL debugging information.

Oracle Database PL/SQL Packages and Types Reference

HTF

Has hypertext functions that generate HTML tags (for example, the HTF.ANCHOR function generates the HTML anchor tag <A>).

Oracle Database PL/SQL Packages and Types Reference

HTP

Has hypertext procedures that generate HTML tags.

Oracle Database PL/SQL Packages and Types Reference

DBMS_PIPE

Lets two or more sessions in the same instance communicate.

Oracle Database PL/SQL Packages and Types Reference

UTL_FILE

Lets PL/SQL programs read and write operating system files.

Oracle Database PL/SQL Packages and Types Reference

UTL_HTTP

Lets PL/SQL programs make Hypertext Transfer Protocol (HTTP) callouts, and access data on the Internet over HTTP.

Oracle Database PL/SQL Packages and Types Reference

UTL_SMTP

Sends electronic mails (emails) over Simple Mail Transfer Protocol (SMTP) as specified by RFC821.

Oracle Database PL/SQL Packages and Types Reference


To display output passed to DBMS_OUTPUT, you need another program, such as SQL*Plus. To see DBMS_OUTPUT output with SQL*Plus, you must first issue the SQL*Plus command SET SERVEROUTPUT ON.

Some subprograms in the packages in Table 1-1 can both accept input and display output, but they cannot accept data directly from the keyboard. To accept data directly from the keyboard, use the SQL*Plus commands PROMPT and ACCEPT.

See Also:

Data Abstraction

Data abstraction lets you work with the essential properties of data without being too involved with details. You can design a data structure first, and then design algorithms that manipulate it.

Topics:

Cursors

A cursor is a pointer to a private SQL area that stores information about processing a specific SQL statement or PL/SQL SELECT INTO statement. You can use the cursor to retrieve the rows of the result set one at a time. You can use cursor attributes to get information about the state of the cursor—for example, how many rows the statement has affected so far. For more information about cursors, see "Cursors".

Composite Variables

A composite variable has internal components, which you can access individually. You can pass entire composite variables to subprograms as parameters. PL/SQL has two kinds of composite variables, collections and records.

In a collection, the internal components are always of the same data type, and are called elements. You access each element by its unique subscript. Lists and arrays are classic examples of collections.

In a record, the internal components can be of different data types, and are called fields. You access each field by its name. A record variable can hold a table row, or some columns from a table row.

For more information about composite variables, see Chapter 5, "PL/SQL Collections and Records."

%ROWTYPE Attribute

The %ROWTYPE attribute lets you declare a record that represents either a full or partial row of a database table or view. For every column of the full or partial row, the record has a field with the same name and data type. If the structure of the row changes, then the structure of the record changes accordingly. For more information about %ROWTYPE, see "%ROWTYPE Attribute".

%TYPE Attribute

The %TYPE attribute lets you declare a data item of the same data type as a previously declared variable or column (without knowing what that type is). If the declaration of the referenced item changes, then the declaration of the referencing item changes accordingly. The %TYPE attribute is particularly useful when declaring variables to hold database values. For more information about %TYPE, see "%TYPE Attribute".

Abstract Data Types

An Abstract Data Type (ADT) consists of a data structure and subprograms that manipulate the data. The variables that form the data structure are called attributes. The subprograms that manipulate the attributes are called methods.

ADTs are stored in the database. Instances of ADTs can be stored in tables and used as PL/SQL variables.

ADTs let you reduce complexity by separating a large system into logical components, which you can reuse.

In the static data dictionary view *_OBJECTS, the OBJECT_TYPE of an ADT is TYPE. In the static data dictionary view *_TYPES, the TYPECODE of an ADT is OBJECT.

For more information about ADTs, see "CREATE TYPE Statement".

Note:

ADTs are also called user-defined types and object types.

See Also:

Oracle Database Object-Relational Developer's Guide for information about ADTs

Control Statements

Control statements are the most important PL/SQL extension to SQL.

PL/SQL has three categories of control statements:

  • Conditional selection statements, which let you run different statements for different data values.

    For more information, see "Conditional Selection Statements".

  • Loop statements, which let you repeat the same statements with a series of different data values.

    For more information, see "LOOP Statements".

  • Sequential control statements, which allow you to go to a specified, labeled statement, or to do nothing.

    For more information, see "Sequential Control Statements".

Conditional Compilation

Conditional compilation lets you customize the functionality in a PL/SQL application without removing source code. For example, you can:

  • Use new features with the latest database release, and disable them when running the application in an older database release.

  • Activate debugging or tracing statements in the development environment, and hide them when running the application at a production site.

For more information, see "Conditional Compilation".

Processing a Query Result Set One Row at a Time

PL/SQL lets you issue a SQL query and process the rows of the result set one at a time. You can use a basic loop, as in Example 1-2, or you can control the process precisely by using individual statements to run the query, retrieve the results, and finish processing.

Example 1-2 Processing Query Result Rows One at a Time

BEGIN
  FOR someone IN (
    SELECT * FROM employees
    WHERE employee_id < 120
    ORDER BY employee_id
  )
  LOOP
    DBMS_OUTPUT.PUT_LINE('First name = ' || someone.first_name ||
                         ', Last name = ' || someone.last_name);
  END LOOP;
END;
/

Result:

First name = Steven, Last name = King
First name = Neena, Last name = Kochhar
First name = Lex, Last name = De Haan
First name = Alexander, Last name = Hunold
First name = Bruce, Last name = Ernst
First name = David, Last name = Austin
First name = Valli, Last name = Pataballa
First name = Diana, Last name = Lorentz
First name = Nancy, Last name = Greenberg
First name = Daniel, Last name = Faviet
First name = John, Last name = Chen
First name = Ismael, Last name = Sciarra
First name = Jose Manuel, Last name = Urman
First name = Luis, Last name = Popp
First name = Den, Last name = Raphaely
First name = Alexander, Last name = Khoo
First name = Shelli, Last name = Baida
First name = Sigal, Last name = Tobias
First name = Guy, Last name = Himuro
First name = Karen, Last name = Colmenares

Architecture of PL/SQL

Topics:

PL/SQL Engine

The PL/SQL compilation and run-time system is an engine that compiles and runs PL/SQL units. The engine can be installed in the database or in an application development tool, such as Oracle Forms.

In either environment, the PL/SQL engine accepts as input any valid PL/SQL unit. The engine runs procedural statements, but sends SQL statements to the SQL engine in the database, as shown in Figure 1-1.

Typically, the database processes PL/SQL units.

When an application development tool processes PL/SQL units, it passes them to its local PL/SQL engine. If a PL/SQL unit contains no SQL statements, the local engine processes the entire PL/SQL unit. This is useful if the application development tool can benefit from conditional and iterative control.

For example, Oracle Forms applications frequently use SQL statements to test the values of field entries and do simple computations. By using PL/SQL instead of SQL, these applications can avoid calls to the database.

PL/SQL Units and Compilation Parameters

A PL/SQL unit is one of these:

  • PL/SQL anonymous block

  • FUNCTION

  • LIBRARY

  • PACKAGE

  • PACKAGE BODY

  • PROCEDURE

  • TRIGGER

  • TYPE

  • TYPE BODY

PL/SQL units are affected by PL/SQL compilation parameters (a category of database initialization parameters). Different PL/SQL units—for example, a package specification and its body—can have different compilation parameter settings.

Table 1-2 summarizes the PL/SQL compilation parameters. To display the values of these parameters for specified or all PL/SQL units, query the static data dictionary view ALL_PLSQL_OBJECT_SETTINGS. For information about this view, see Oracle Database Reference.

Table 1-2 PL/SQL Compilation Parameters

Parameter Description

PLSCOPE_SETTINGS

Controls the compile-time collection, cross-reference, and storage of PL/SQL source code identifier data. Used by the PL/Scope tool (see Oracle Database Advanced Application Developer's Guide).

For more information about PLSCOPE_SETTINGS, see Oracle Database Reference.

PLSQL_CCFLAGS

Enables you to control conditional compilation of each PL/SQL unit independently.

For more information about PLSQL_CCFLAGS, see "How Conditional Compilation Works" and Oracle Database Reference.

PLSQL_CODE_TYPE

Specifies the compilation mode for PL/SQL units—INTERPRETED (the default) or NATIVE. For information about which mode to use, see "Determining Whether to Use PL/SQL Native Compilation".

If the optimization level (set by PLSQL_OPTIMIZE_LEVEL) is less than 2:

  • The compiler generates interpreted code, regardless of PLSQL_CODE_TYPE.

  • If you specify NATIVE, the compiler warns you that NATIVE was ignored.

For more information about PLSQL_CODE_TYPE, see Oracle Database Reference.

PLSQL_OPTIMIZE_LEVEL

Specifies the optimization level at which to compile PL/SQL units (the higher the level, the more optimizations the compiler tries to make).

PLSQL_OPTIMIZE_LEVEL=1 instructs the PL/SQL compiler to generate and store code for use by the PL/SQL debugger.

For more information about PLSQL_OPTIMIZE_LEVEL, see "PL/SQL Optimizer" and Oracle Database Reference.

PLSQL_WARNINGS

Enables or disables the reporting of warning messages by the PL/SQL compiler, and specifies which warning messages to show as errors.

For more information about PLSQL_WARNINGS, see "Compile-Time Warnings" and Oracle Database Reference.

NLS_LENGTH_SEMANTICS

Enables you to create CHAR and VARCHAR2 columns using either byte-length or character-length semantics.

For more information about byte and character length semantics, see "CHAR and VARCHAR2 Variables".

For more information about NLS_LENGTH_SEMANTICS, see Oracle Database Reference.


Note:

The compiler parameter PLSQL_DEBUG, which specifies whether to compile PL/SQL units for debugging, is deprecated. To compile PL/SQL units for debugging, specify PLSQL_OPTIMIZE_LEVEL=1.

The compile-time values of the parameters in Table 1-2 are stored with the metadata of each stored PL/SQL unit, which means that you can reuse those values when you explicitly recompile the unit. (A stored PL/SQL unit is created with one of the "CREATE [OR REPLACE] Statements". An anonymous block is not a stored PL/SQL unit.)

To explicitly recompile a stored PL/SQL unit and reuse its parameter values, you must use an ALTER statement with both the COMPILE clause and the REUSE SETTINGS clause. For more information about REUSE SETTINGS, see "compiler_parameters_clause". (All ALTER statements have this clause. For a list of ALTER statements, see "ALTER Statements".)