Exception Handling in PL/SQL: A Comprehensive Guide with Examples

Exception handling in PL/SQL has an important role in ensuring that programmers can manage unexpected errors or exceptional conditions during their programs’ execution. With exception handling developers have various strategies available when working to develop applications with minimal errors. By making use of features such as try-catch blocks and exception propagation via PL/SQL. Programmers can detect and respond to specific issues more efficiently while simultaneously improving performance metrics related to overall quality control.

In this blog, we will explore the types of exceptions in PL/SQL, the syntax for exception handling, various techniques, and best practices.

What is Exception Handling in PL/SQL?

Exception handling in PL/SQL is a valuable mechanism for dealing with errors or exceptional conditions that may occur while executing a program. It allows developers to capture and address specific exceptions. Providing a well-organized approach to dealing with errors and ensuring the program continues to run smoothly. With the use of exception handlers, developers can gracefully manage errors, carry out necessary cleanup operations and effectively control the program’s behavior when exceptions arise.
Having a foundation in SQL is very useful when working on PL/SQL. It is a good practice to learn SQL before you move to PL/SQL.

Types of Exceptions in PL/SQL

Handling PL/SQL exceptions during development involves exploring numerous options. One possibility is utilizing Oracle’s existing tools; alternatively, developers might opt for creating customized mechanisms for addressing specific error management requirements. 

There are many exceptions related to PL/SQL. Some of them include:

Type of Exception	Exception Name	Description
Predefined Exceptions	NO_DATA_FOUND	Raised when a SELECT statement returns no rows.
	TOO_MANY_ROWS	Raised when a SELECT statement returns multiple rows.
	DUP_VAL_ON_INDEX	Raised when a unique index or primary key constraint is violated.
User-Defined Exceptions	User-Defined	Exceptions are defined by developers to handle specific error conditions unique to an application.
System Exceptions	ACCESS_INTO_NULL	Raised when a null object reference is accessed.
	CASE_NOT_FOUND	Raised when no case statement is found for a given value.
	COLLECTION_IS_NULL	Raised when a program attempts to access a collection that has not been initialized.
Named System Exceptions	ZERO_DIVIDE	Raised when a division or modulus operation is performed with zero as the divisor.
	TIMEOUT_ON_RESOURCE	Raised when a timeout occurs while waiting for a resource.

By using these exceptions, developers can promptly and precisely deal with various precision-related glitches while keeping in mind each application’s individualized standards.

Exception Handling Syntax in PL/SQL

A special case dealing with PL/SQL is improved through the use of an TRY-CATCH style language structure. The code block among Start and Exception is where the error code has to be contained. Whenever an exception occurs while executing inside this limit, program control will move consistently to its related Special case block.

Here’s an example of the syntax for a PL/SQL block with a TRY-CATCH style structure:

BEGIN
  -- Code block where the error handling code is contained
  -- ...
  -- Your PL/SQL statements here
  
EXCEPTION
  WHEN exception1 THEN
    -- Code to handle exception1
    
  WHEN exception2 THEN
    -- Code to handle exception2
    
  WHEN others THEN
    -- Code to handle all other exceptions
  
END;

PL/SQL works with engineers to deal with a different scope of exemptions. It enables them to:
Manage explicit special cases, give exact arrangements as well as deal with various special cases that surface inside a solitary block in disparate structures.
Settle exemptions to consider flowing special cases dealing with, where an external block gets and handles exemptions brought up in an inward block.

The RAISE proclamation presented by PL/SQL is a helpful tool for re-raising recently gotten exemptions inside a special case controller. It demonstrates useful while sending exemptions for the climate that has conjured it or while raising special cases for upper-level blocks so a bound-together blunder control component can be laid out.

Here is the syntax for using the RAISE statement in PL/SQL to re-raise exceptions:

DECLARE
  -- Exception declaration
  exception_name EXCEPTION;
  
BEGIN
  -- Exception handling block
  BEGIN
    -- Code that may raise an exception
    NULL;
    
    EXCEPTION
      WHEN exception_name THEN
        -- Exception handling code
        -- ...
        -- Optionally, re-raise the exception
        RAISE;
  END;
  
EXCEPTION
  WHEN exception_name THEN
    -- Exception handling code for the re-raised exception
    -- ...
    
END;

Exception Handling Techniques

A commonly utilized method called exception propagation is implemented to transfer an exception to a higher-level block or the invoking environment. Oracle permits the usage of the EXCEPTION_INIT pragma, which permits one to affiliate a user-specified exception with a specific error code. This technique enables us to handle specific errors at a higher level or provide custom error messages for predefined exceptions.

Efficient exception handling in PL/SQL programs can be achieved by considering several best practices. It is critical to log and report errors effectively, ensuring detailed information for testing and problem-resolution purposes. Furthermore, implementing graceful degradation and error recovery mechanisms can help handle abnormal scenarios without impacting the program’s normal flow. 

To ensure the smooth operation of your program, limiting the use of unnecessarily complex exception handling in SQL is recommended for better performance efficiency. On top of that, utilizing effective exception-handling methods within loops and cursors enhances both error detection accuracy and resource management optimization.

Exception Handling Examples

To better understand exception handling in PL/SQL, let’s explore some practical examples.

Example 1 – Handling division by zero exception

BEGIN
  DECLARE
    numerator NUMBER := 10;
    denominator NUMBER := 0;
    result NUMBER;
  BEGIN
    result:= numerator / denominator;
    DBMS_OUTPUT.PUT_LINE('Result: ' || result);
  EXCEPTION
    WHEN ZERO_DIVIDE THEN
      DBMS_OUTPUT.PUT_LINE('Error: Division by zero');
  END;
END;

In this example code, there is an attempt to divide a numerically zero value which usually triggers a ZERO_DIVIDE exception. Nonetheless, implementing exception handling allows us to intercept this matter and respond gracefully by presenting an error message.

Example 2 – Handling unique constraint violation exception

BEGIN
  DECLARE
    emp_id NUMBER := 100;
    emp_name VARCHAR2(50) := 'John Doe';
  BEGIN
    INSERT INTO employees (id, name) VALUES (emp_id, emp_name);
    COMMIT;
    DBMS_OUTPUT.PUT

_LINE('Employee added successfully');
  EXCEPTION
    WHEN DUP_VAL_ON_INDEX THEN
      DBMS_OUTPUT.PUT_LINE('Error: Employee ID already exists');
  END;
END;

In this example, we add new personnel to our “employees” table, we run into a problem when the chosen employee ID already exists in the table, violating its unique constraint. This error produces a DUP_VAL_ON_INDEX exception. Through precise handling, we can address this issue and ensure that any error messages given carry significant weight.

Exception Handling Considerations

Exception handling in PL/SQL requires consideration of various factors, these include:

• Strike a balance- Implementing PL/SQL method may result in slight performance overheads because of additional processing necessary during an exception. Therefore, it is essential to strike a balance between effectively managing exceptions and maintaining optimal system performance.
• Dealing with distributed transactions– It demands the proper handling of exceptions to maintain data consistency and integrity across multiple database nodes. It is important to manage errors effectively while maintaining efficiency during bulk operations such as bulk inserts or updates by employing careful exception handling.
• Handling exceptions in triggers and stored procedures – This should be considered a prime concern. By addressing exceptions swiftly within these database constructs you can alleviate concerns about data integrity issues while having greater confidence in the consistency of your applications’ behavior.

Conclusion

The role of exception handling in PL/SQL is to create reliable applications. Managing potential errors appropriately guarantees that developers can keep their programs functioning seamlessly without any unexpected errors. Accompanied by practical examples, we have demonstrated the power and flexibility of exception handling. By implementing these techniques and following best practices, developers can enhance the quality and stability of their PL/SQL programs.