Raising Error Conditions with MySQL SIGNAL / RESIGNAL Statements

Raising Error Conditions with MySQL SIGNAL and RESIGNAL Statements

The SIGNAL and RESIGNAL statements in MySQL allow you to explicitly raise error or warning conditions during the execution of stored procedures, triggers, or events. These statements are useful for custom error handling and improving the clarity of database logic by reporting specific error messages or SQLSTATE values.

The SIGNAL Statement

The SIGNAL statement is used to raise an error or warning condition explicitly. It allows you to define custom SQLSTATE codes and error messages.

Syntax

SIGNAL SQLSTATE 'sqlstate_value'
    SET error_property = value [, error_property = value ...];

• sqlstate_value: A 5-character alphanumeric string that represents a standard or custom SQLSTATE code.
  • Example: '45000' is commonly used for generic custom errors.
• error_property: Specifies additional details about the error.
  • Common properties:
    • MESSAGE_TEXT: A descriptive error message.
    • MYSQL_ERRNO: A MySQL error code (optional).
    • CONDITION_IDENTIFIER: A symbolic name for the condition.
    • MESSAGE_LENGTH: Length of the error message (optional).

Basic Example: Raising a Custom Error

DELIMITER $$

CREATE PROCEDURE raise_error_example()
BEGIN
    -- Custom error raised
    SIGNAL SQLSTATE '45000'
        SET MESSAGE_TEXT = 'A custom error has occurred.', MYSQL_ERRNO = 1001;
END$$

DELIMITER ;

Execution

CALL raise_error_example();

Output

ERROR 1644 (45000): A custom error has occurred.

• 1644: A MySQL-specific error code for SIGNAL.
• 45000: A custom SQLSTATE value.

The RESIGNAL Statement

The RESIGNAL statement is used to re-raise an existing error condition, optionally modifying its details. It is typically used in error handlers to propagate or customize errors.

Syntax

RESIGNAL [SQLSTATE 'sqlstate_value']
    [SET error_property = value [, error_property = value ...]];

• When no SQLSTATE or properties are provided, RESIGNAL propagates the original error unchanged.

Example: Propagating Errors with RESIGNAL

Stored Procedure with Error Handling

DELIMITER $$

CREATE PROCEDURE handle_error_example()
BEGIN
    DECLARE EXIT HANDLER FOR SQLEXCEPTION
    BEGIN
        -- Log the original error
        INSERT INTO error_log (error_message, error_time)
        VALUES ('An error occurred', NOW());

        -- Propagate the error with additional context
        RESIGNAL SQLSTATE '45000'
            SET MESSAGE_TEXT = 'An error occurred during procedure execution.';
    END;

    -- Intentionally cause an error
    INSERT INTO non_existing_table VALUES (1);
END$$

DELIMITER ;

Execution

CALL handle_error_example();

Output

ERROR 1644 (45000): An error occurred during procedure execution.

Use Cases for SIGNAL and RESIGNAL

1. Custom Error Messages:

   • Provide meaningful error messages to users or applications.

2. Validation Rules:

   • Enforce specific business rules by raising errors when conditions are not met.
   • Example: Prevent inserting invalid data.

3. Error Propagation:

   • Use RESIGNAL to log errors and re-raise them with additional details or context.

Advanced Examples

1. Input Validation with SIGNAL

DELIMITER $$

CREATE PROCEDURE validate_input(IN input_value INT)
BEGIN
    IF input_value IS NULL OR input_value <= 0 THEN
        SIGNAL SQLSTATE '45000'
            SET MESSAGE_TEXT = 'Input value must be a positive integer.', MYSQL_ERRNO = 1002;
    END IF;

    -- Continue processing if valid
    INSERT INTO valid_data (value) VALUES (input_value);
END$$

DELIMITER ;

Execution

CALL validate_input(-5);

Output

ERROR 1644 (45000): Input value must be a positive integer.

2. Logging and Propagating Errors with RESIGNAL

DELIMITER $$

CREATE PROCEDURE log_and_resignal_error()
BEGIN
    DECLARE EXIT HANDLER FOR SQLEXCEPTION
    BEGIN
        -- Log the error
        INSERT INTO error_log (error_message, error_time)
        VALUES ('An SQL error occurred', NOW());

        -- Re-raise the error with additional context
        RESIGNAL;
    END;

    -- Cause an error
    SELECT * FROM non_existing_table;
END$$

DELIMITER ;

Execution

CALL log_and_resignal_error();

Output

ERROR 1146 (42S02): Table 'your_database.non_existing_table' doesn't exist

3. Custom SQLSTATE in RESIGNAL

DELIMITER $$

CREATE PROCEDURE resignal_with_custom_message()
BEGIN
    DECLARE EXIT HANDLER FOR SQLEXCEPTION
    BEGIN
        -- Modify and propagate the error
        RESIGNAL SQLSTATE '45000'
            SET MESSAGE_TEXT = 'A custom error occurred. Please contact support.', MYSQL_ERRNO = 2001;
    END;

    -- Cause an error
    INSERT INTO non_existing_table VALUES (1);
END$$

DELIMITER ;

Execution

CALL resignal_with_custom_message();

Output

ERROR 1644 (45000): A custom error occurred. Please contact support.

Best Practices for SIGNAL and RESIGNAL

1. Meaningful Messages:

   • Always provide clear and actionable error messages.

2. Custom SQLSTATE Codes:

   • Use 45000 for general-purpose custom errors.
   • Avoid overlapping with standard SQLSTATE codes unless reusing specific conditions intentionally.

3. Error Logging:

   • Log errors in a dedicated table for debugging and auditing.

4. Transaction Management:

   • Combine SIGNAL and RESIGNAL with transactions (START TRANSACTION, ROLLBACK, COMMIT) to maintain data consistency.

5. Test Thoroughly:

   • Simulate various error scenarios to ensure your error-handling logic works as expected.

Conclusion

The SIGNAL and RESIGNAL statements in MySQL are powerful tools for creating robust and user-friendly stored procedures. They enable custom error reporting, validation, and propagation, making database logic more transparent and maintainable. With proper use, you can ensure that your stored procedures handle errors gracefully and provide meaningful feedback.

Let me know if you need further clarification or additional examples!