Handling Database Exceptions in Proc*C

In database-centric C applications, it's crucial to handle exceptions that may occur during database operations. Exceptions can be caused by various factors, such as database errors, data integrity issues, or connectivity problems. In this tutorial, we will explore how to handle database exceptions in Proc*C and explain the steps to effectively manage errors in your C applications when working with embedded SQL.

1. Handling Exceptions in Proc*C

In Proc*C, you can handle database exceptions using the EXEC SQL WHENEVER statement. This statement allows you to specify what action should be taken when specific types of exceptions are raised. The common types of exceptions that you can handle include SQLERROR, SQLWARNING, and NOT FOUND.

Let's take an example of a simple SQL query to retrieve employee details from the employees table. We will use the WHENEVER SQLERROR statement to handle any database errors that may occur during the execution of the query:

      /* EXEC SQL BEGIN DECLARE SECTION; */
      int employee_id = 1001;
      char employee_name[50];
      /* EXEC SQL END DECLARE SECTION; */
  /* EXEC SQL WHENEVER SQLERROR GOTO error_handling; */
  
  /* EXEC SQL SELECT employee_name INTO :employee_name
     FROM employees WHERE emp_id = :employee_id;
  END-EXEC; */

  /* EXEC SQL WHENEVER NOT FOUND CONTINUE; */
  
  /* EXEC SQL INSERT INTO audit_table VALUES(:employee_id);
  END-EXEC; */

  /* EXEC SQL COMMIT WORK;
  END-EXEC; */

  /* EXEC SQL WHENEVER SQLERROR GOTO 0; */
  /* EXEC SQL WHENEVER NOT FOUND GOTO 0; */

  printf("Employee name: %s\n", employee_name);
  printf("Employee details successfully inserted into audit table.\n");
  return 0;

error_handling:
  printf("Database error occurred. Please try again later.\n");
  return -1;

In this example, we declare an integer variable employee_id and a character array employee_name to hold the employee details. We use the WHENEVER SQLERROR statement to jump to the error_handling label if any database error occurs during the execution of the SQL query. Additionally, we use the WHENEVER NOT FOUND CONTINUE statement to continue the program execution if the SQL query does not find any matching rows. After the successful execution of the INSERT statement, we commit the transaction using the COMMIT WORK statement.

2. Handling Exceptions Effectively

To handle exceptions effectively in Proc*C, consider the following tips:

• Always check the return codes of database operations to identify and handle exceptions.
• Use meaningful error messages to provide useful information to users in case of errors.
• Perform necessary rollback operations to ensure data consistency when exceptions occur during a transaction.
• Limit the use of generic exception handling and use specific exception handlers for different scenarios.

3. Frequently Asked Questions (FAQs)

• Q: What is the difference between SQLERROR and SQLWARNING?
  A: SQLERROR is raised when a database error occurs, such as syntax errors or integrity violations. SQLWARNING is raised for less severe issues, such as data truncation.
• Q: How can I handle exceptions other than SQLERROR and SQLWARNING?
  A: You can use the WHENEVER statement with other exception types, such as NOT FOUND or NOT SQL in operation, depending on your database system and the available exception types.
• Q: Can I handle exceptions at the application level instead of using WHENEVER?
  A: Yes, you can handle exceptions at the application level by checking the return codes of database operations and implementing custom exception handling logic.
• Q: Is it necessary to use the COMMIT statement after a successful database operation?
  A: It depends on your application's transaction requirements. In some cases, you may want to commit changes to the database after a successful operation, while in others, you may want to keep the changes pending until other operations are completed.
• Q: How can I log database exceptions for debugging purposes?
  A: You can log database exceptions to a log file or a database table using custom error handling procedures or logging libraries in your C application.

4. Summary

Handling database exceptions in Proc*C is essential for ensuring the reliability and integrity of your C applications that interact with a database. By using the WHENEVER statement and implementing effective exception handling strategies, you can gracefully handle errors and provide meaningful feedback to users when database exceptions occur.