Error Handling (Transact-SQL)

This topic provides information about error handling in Transact-SQL (T-SQL). Effective error handling is crucial for building robust and reliable database applications. T-SQL offers several mechanisms to detect, report, and manage errors that occur during the execution of your scripts and stored procedures.

Understanding Errors in T-SQL

Errors in T-SQL can be categorized as:

• Syntax Errors: Errors related to the structure and grammar of T-SQL statements. These are typically caught by the SQL Server engine before execution.
• Runtime Errors: Errors that occur during the execution of valid T-SQL code. These can be due to various reasons like invalid data, constraint violations, resource issues, or logical problems.
• Errors Raised by the User: Errors explicitly signaled using the RAISERROR statement.

Mechanisms for Error Handling

1. @@ERROR System Function (Legacy)

The @@ERROR global variable returns the error number of the last Transact-SQL statement executed. A return value of 0 indicates that the statement executed successfully. A non-zero value indicates an error.

Note: While @@ERROR is still supported for backward compatibility, it is recommended to use the TRY...CATCH construct for new development, as it provides more robust and structured error handling.

IF @@ERROR <> 0
BEGIN
    -- Handle the error here
    PRINT 'An error occurred!'
END

2. RAISERROR Statement

The RAISERROR statement is used to generate custom error messages and return them to the client application. You can define your own error numbers and severity levels.

RAISERROR ('User-defined error message.', 16, 1) WITH LOG

Parameters:

• message_id or message_string: The message to return.
• severity: A value from 0 to 25 that indicates the severity of the error. Severity levels 19 through 25 are fatal and cause the connection to terminate.
• state: An integer from 1 to 255.
• WITH LOG: Logs the error message to the SQL Server error log and the Windows Application log.

3. TRY...CATCH Construct (Recommended)

The TRY...CATCH construct provides a structured way to handle errors in T-SQL. The code that might cause an error is placed within the TRY block. If an error occurs within the TRY block, execution immediately transfers to the CATCH block.

BEGIN TRY
    -- Code that might raise an error
    SELECT 1 / 0
END TRY
BEGIN CATCH
    -- Error handling code
    PRINT 'An error occurred in the TRY block.'
    PRINT 'Error Number: ' + CAST(ERROR_NUMBER() AS VARCHAR(10))
    PRINT 'Error Message: ' + ERROR_MESSAGE()
    PRINT 'Error Severity: ' + CAST(ERROR_SEVERITY() AS VARCHAR(10))
    PRINT 'Error State: ' + CAST(ERROR_STATE() AS VARCHAR(10))
    PRINT 'Error Procedure: ' + ISNULL(ERROR_PROCEDURE(), '(Not from a stored procedure)')
    PRINT 'Error Line: ' + CAST(ERROR_LINE() AS VARCHAR(10))

    -- Optionally, re-raise the error or return a custom error
    -- RAISERROR('Custom error message for the caller.', 16, 1)
END CATCH

Within the CATCH block, you can use the following error-handling functions:

• ERROR_NUMBER(): Returns the error number.
• ERROR_MESSAGE(): Returns the complete text of the error message.
• ERROR_SEVERITY(): Returns the severity level of the error.
• ERROR_STATE(): Returns the state number of the error.
• ERROR_PROCEDURE(): Returns the name of the stored procedure or function where the error occurred.
• ERROR_LINE(): Returns the line number in the batch or stored procedure where the error occurred.

Best Practices for Error Handling

• Always use TRY...CATCH for new T-SQL development.
• Log errors with relevant details (error number, message, severity, line number, timestamp) to aid in debugging.
• Consider how errors should propagate to the calling application. You may want to re-raise the error, return a custom error message, or perform specific cleanup actions.
• Use appropriate severity levels for RAISERROR to indicate the impact of the error.
• Handle specific error numbers if you need to react differently to different types of errors.