Introduction to Exception in PL/SQL

Topic Introduction: In PL/SQL, an exception is an error or an abnormal condition that occurs during the execution of a program. Exceptions allow you to handle errors and unexpected situations in a structured and controlled manner, preventing your program from crashing or producing incorrect results when problems arise.

PL/SQL exceptions are categorized into two main types:

Predefined Exceptions: These are exceptions that Oracle has already defined for common error conditions. Examples of predefined exceptions include NO_DATA_FOUND, TOO_MANY_ROWS, and INVALID_NUMBER. These exceptions are raised automatically by Oracle when certain errors occur, and you can catch and handle them in your PL/SQL code using exception handling blocks.

User-Defined Exceptions: These are exceptions that you define in your PL/SQL code to handle specific errors that are relevant to your application. User-defined exceptions allow you to customize error handling and provide more meaningful error messages to users.

Exception handling in PL/SQL involves using the BEGIN ... EXCEPTION ... END block structure. 

Here's a simple example of how exception handling works:

DECLARE

    v_balance NUMBER := 100;

BEGIN

    IF v_balance < 0 THEN

        RAISE_APPLICATION_ERROR(-20001, 'Negative balance not allowed.');

    END IF;

EXCEPTION

    WHEN OTHERS THEN

        DBMS_OUTPUT.PUT_LINE('An error occurred: ' || SQLERRM);

END;

In this example, if the balance becomes negative, the program raises a user-defined exception using the RAISE_APPLICATION_ERROR procedure. The EXCEPTION block catches any exceptions that occur within the BEGIN block. The WHEN OTHERS clause catches all exceptions not explicitly handled elsewhere.

It's important to note that exceptions should be handled gracefully to provide clear error messages and ensure that your program continues to run smoothly even in the presence of errors. Unhandled exceptions can lead to unexpected behavior and application crashes.