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.
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
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 functions can be declared and defined in the
WITH clauses of SQL
SELECT statements (see Oracle Database SQL Language Reference).
PL/SQL supports both static and dynamic SQL. Static SQL is SQL whose full text is known at compile time. Dynamic SQL is SQL whose full text is not known until run time. Dynamic SQL lets you make your applications more flexible and versatile. For more information, see Chapter 6, "PL/SQL Static SQL" and Chapter 7, "PL/SQL Dynamic SQL".
When you embed a SQL
SELECT statement directly in your PL/SQL code, the PL/SQL compiler turns the variables in the
VALUES clauses into bind variables (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 variables automatically when you use dynamic SQL, but you can use them with dynamic SQL by specifying them explicitly (for details, see "EXECUTE IMMEDIATE Statement").
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".
The PL/SQL compiler has an optimizer that can rearrange code for better performance. For more information about the optimizer, see "PL/SQL Optimizer".
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".
PL/SQL stored subprograms increase scalability by centralizing application processing on the database server. The shared memory facilities of the shared server let Oracle Database 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.
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".
PL/SQL supports object-oriented programming with "Abstract Data Types".
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 runtime errors.
You can break complex problems into easily understandable subprograms, which you can reuse in multiple applications.
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".
A PL/SQL block is defined by the keywords
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.
<< 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:
Syntax checking: PL/SQL syntax is checked, and a parse tree is generated.
Semantic checking: Type checking and further processing on the parse tree.
Note:An anonymous block is a SQL statement.
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".
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".
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.
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
For more information about triggers, see Chapter 9, "PL/SQL Triggers."
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.
Lets PL/SQL blocks, subprograms, packages, and triggers display output. Especially useful for displaying PL/SQL debugging information.
Has hypertext functions that generate HTML tags (for example, the
Has hypertext procedures that generate HTML tags.
Lets two or more sessions in the same instance communicate.
Lets PL/SQL programs read and write operating system files.
Lets PL/SQL programs make Hypertext Transfer Protocol (HTTP) callouts, and access data on the Internet over HTTP.
Sends electronic mails (emails) over Simple Mail Transfer Protocol (SMTP) as specified by RFC821.
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
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
A cursor is a pointer to a private SQL area that stores information about processing a specific SQL statement or PL/SQL
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".
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 index. 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 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 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".
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
OBJECT_TYPE of an ADT is
TYPE. In the static data dictionary view
TYPECODE of an ADT is
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 (which it calls object types)
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 lets you customize the functionality in a PL/SQL application without removing source text. 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".
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.
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; /
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
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 anonymous block
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.
Controls the compile-time collection, cross-reference, and storage of PL/SQL source text identifier data. Used by the PL/Scope tool (see Oracle Database Development Guide).
For more information about
Lets you control conditional compilation of each PL/SQL unit independently.
Specifies the compilation mode for PL/SQL units—
If the optimization level (set by
For more information about
Specifies the optimization level at which to compile PL/SQL units (the higher the level, the more optimizations the compiler tries to make).
Enables or disables the reporting of warning messages by the PL/SQL compiler, and specifies which warning messages to show as errors.
Lets you create
For more information about byte and character length semantics, see "CHAR and VARCHAR2 Variables".
For more information about
Specifies whether the 12.1 PL/SQL compiler can use wrapped packages that were compiled with the 9.2 PL/SQL compiler. The default value is
For more information about wrapped packages, see Appendix A, "PL/SQL Source Text Wrapping."
For more information about
Note:The compilation parameter
PLSQL_DEBUG, which specifies whether to compile PL/SQL units for debugging, is deprecated. To compile PL/SQL units for debugging, specify
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
SETTINGS clause. For more information about
SETTINGS, see "compiler_parameters_clause". (All
ALTER statements have this clause. For a list of
ALTER statements, see "ALTER Statements".)