PL/SQL Architecture

CREATE TYPE Bank_Account AS OBJECT ( acct_number INTEGER(5),

balance REAL,

status VARCHAR2(10),

MEMBER PROCEDURE open (amount IN REAL), MEMBER PROCEDURE verify_acct (num IN INTEGER),

MEMBER PROCEDURE close (num IN INTEGER, amount OUT REAL), MEMBER PROCEDURE deposit (num IN INTEGER, amount IN REAL), MEMBER PROCEDURE withdraw (num IN INTEGER, amount IN REAL), MEMBER FUNCTION curr_bal (num IN INTEGER) RETURN REAL

);

Error Handling

PL/SQL makes it easy to detect and process error conditions known as exceptions. When an error occurs, an exception is raised: normal execution stops and control transfers to special exception-handling code, which comes at the end of any PL/SQL block. Each different exception is processed by a particular exception handler.

Predefined exceptions are raised automatically for certain common error conditions involving variables or database operations. For example, if you try to divide a number by zero, PL/SQL raises the predefined exception ZERO_DIVIDE automatically.

You can declare exceptions of your own, for conditions that you decide are errors, or to correspond to database errors that normally result in ORAerror messages. When you detect a user-defined error condition, you execute a RAISE statement. The following example computes the bonus earned by a salesperson. The bonus is based on salary and commission. If the commission is null, you raise the exception comm_missing.

DECLARE

comm_missing EXCEPTION; -- declare exception BEGIN

IF commission IS NULL THEN

RAISE comm_missing; -- raise exception END IF;

bonus := (salary * 0.10) + (commission * 0.15); EXCEPTION

WHEN comm_missing THEN ... -- process the exception

PL/SQL Architecture

The PL/SQL compilation and run-time system is an engine that compiles and executes PL/SQL blocks and subprograms. The engine can be installed in an Oracle server or in an application development tool such as Oracle Forms or Oracle Reports.

In either environment, the PL/SQL engine accepts as input any valid PL/SQL block or subprogram. Figure 1–3 shows the PL/SQL engine processing an anonymous block. The PL/SQL engine executes procedural statements but sends SQL statements to the SQL engine in the Oracle database.

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PL/SQL Architecture

SQL Statement Executor

Oracle Server

In the Oracle Database Server

Typically, the Oracle database server processes PL/SQL blocks and subprograms.

Anonymous Blocks

Anonymous PL/SQL blocks can be submitted to interactive tools such as SQL*Plus and Enterprise Manager, or embedded in an Oracle Precompiler or OCI program. At run time, the program sends these blocks to the Oracle database, where they are compiled and executed.

Stored Subprograms

Subprograms can be compiled and stored in an Oracle database, ready to be executed. Once compiled, it is a schema object known as a stored procedure or stored function, which can be referenced by any number of applications connected to that database.

Stored subprograms defined within a package are known as packaged subprograms. Those defined independently are called standalone subprograms.

Subprograms nested inside other subprograms or within a PL/SQL block are known as local subprograms, which cannot be referenced by other applications and exist only inside the enclosing block.

Stored subprograms are the key to modular, reusable PL/SQL code. Wherever you might use a JAR file in Java, a module in Perl, a shared library in C++, or a DLL in Visual Basic, you should use PL/SQL stored procedures, stored functions, and packages.

You can call stored subprograms from a database trigger, another stored subprogram, an Oracle Precompiler or OCI application, or interactively from SQL*Plus or Enterprise Manager. You can also configure a web server so that the HTML for a web page is generated by a stored subprogram, making it simple to provide a web interface for data entry and report generation.

For example, you might call the standalone procedure create_dept from SQL*Plus as follows:

SQL> CALL create_dept('FINANCE', 'NEW YORK');

Overview of PL/SQL 1-13

PL/SQL Architecture

Subprograms are stored in a compact compiled form. When called, they are loaded and processed immediately. Subprograms take advantage of shared memory, so that only one copy of a subprogram is loaded into memory for execution by multiple users.

Database Triggers

A database trigger is a stored subprogram associated with a database table, view, or event. The trigger can be called once, when some event occurs, or many times, once for each row affected by an INSERT, UPDATE, or DELETE statement. The trigger can be called after the event, to record it or take some followup action. Or, the trigger can be called before the event to prevent erroneous operations or fix new data so that it conforms to business rules. For example, the following table-level trigger fires whenever salaries in the emp table are updated:

CREATE TRIGGER audit_sal

AFTER UPDATE OF sal ON emp

FOR EACH ROW

BEGIN

INSERT INTO emp_audit VALUES ...

END;

The executable part of a trigger can contain procedural statements as well as SQL data manipulation statements. Besides table-level triggers, there are instead-of triggers for views and system-event triggers for schemas. For more information, see Oracle Database Application Developer's Guide - Fundamentals.

In Oracle Tools

An application development tool that contains the PL/SQL engine can process PL/SQL blocks and subprograms. The tool passes the blocks to its local PL/SQL engine. The engine executes all procedural statements inside the application and sends only SQL statements to the database. Most of the work is done inside the application, not on the database server. If the block contains no SQL statements, the application executes the entire block. This is useful if your application can benefit from conditional and iterative control.

Frequently, Oracle Forms applications use SQL statements to test the value of field entries or to do simple computations. By using PL/SQL instead, you can avoid calls to the database. You can also use PL/SQL functions to manipulate field entries.

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