T-SQL Error Handling

This section covers how to effectively handle errors in Transact-SQL (T-SQL) scripts and stored procedures to ensure application robustness and data integrity.

Error Handling Concepts

Error handling in T-SQL involves anticipating potential issues during query execution and providing mechanisms to gracefully manage them. This prevents unexpected application crashes and allows for informative logging or alternative actions.

• Error Numbers and Severity: Understanding the built-in error codes and severity levels provided by SQL Server.
• Raising Errors: Using the RAISERROR statement to generate custom error messages.
• Error Handling Blocks: Implementing structured error handling using TRY...CATCH blocks.

TRY...CATCH Blocks

The TRY...CATCH construct is the primary method for structured error handling in T-SQL. It allows you to enclose a block of T-SQL code that might raise an error within a TRY block. If any error occurs within the TRY block, execution immediately transfers to the corresponding CATCH block.

BEGIN TRY
    -- Code that might cause an error
    SELECT 1 / 0;
END TRY
BEGIN CATCH
    -- Error handling logic
    PRINT 'An error occurred.';
    SELECT
        ERROR_NUMBER() AS ErrorNumber,
        ERROR_SEVERITY() AS ErrorSeverity,
        ERROR_STATE() AS ErrorState,
        ERROR_PROCEDURE() AS ErrorProcedure,
        ERROR_LINE() AS ErrorLine,
        ERROR_MESSAGE() AS ErrorMessage;
END CATCH
            

Error Handling Functions within CATCH

Inside a CATCH block, several system functions are available to retrieve details about the error that occurred:

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

Raising Errors with RAISERROR

The RAISERROR statement allows you to generate custom error messages that can be returned to the caller. This is useful for signaling specific conditions or invalid data.

IF EXISTS (SELECT 1 FROM YourTable WHERE SomeColumn IS NULL)
BEGIN
    RAISERROR('Error: SomeColumn cannot be NULL. Please check your data.', 16, 1);
END
            

Common Error Scenarios and Handling

Here are some common situations where error handling is crucial:

• Data Validation: Ensuring data meets specific criteria before insertion or update.
• Concurrency Issues: Handling potential deadlocks or optimistic concurrency conflicts.
• Resource Issues: Managing errors related to insufficient disk space or memory.
• Integrity Constraints: Catching violations of primary keys, foreign keys, or unique constraints.