TRY/CATCH Error Handling in SQL Server: THROW vs RAISERROR

Production T-SQL needs to fail gracefully, not just fail. An unhandled error in the middle of a multi-step operation can leave data in a half-finished state — TRY/CATCH, paired with transactions (covered in full in Chapter 9), is how you prevent that.

The Pattern

BEGIN TRY
    UPDATE dbo.Product SET stock_qty = stock_qty - 10 WHERE product_id = 2;
    IF (SELECT stock_qty FROM dbo.Product WHERE product_id = 2) < 0
        THROW 51000, 'Stock quantity cannot go negative.', 1;
END TRY
BEGIN CATCH
    PRINT 'Error caught: ' + ERROR_MESSAGE();
    PRINT 'Error number: ' + CAST(ERROR_NUMBER() AS VARCHAR(10));
    PRINT 'Error line: ' + CAST(ERROR_LINE() AS VARCHAR(10));
END CATCH;

Code inside BEGIN TRY ... END TRY runs normally. The instant any statement in that block raises an error, execution jumps immediately to BEGIN CATCH ... END CATCH — the rest of the TRY block is skipped entirely, similar to try/catch in C#, Java, or Python, but with SQL-Server-specific error inspection functions.

THROW vs RAISERROR

THROW is preferred — it's simpler, and with no arguments inside a CATCH block, it correctly preserves the original error's number, severity, and state when re-thrown. Reach for RAISERROR only when you need its legacy formatting (%s/%d placeholders) or must support very old SQL Server versions.

-- Re-throwing the ORIGINAL caught error, unchanged, after logging it:
BEGIN TRY
    SELECT 1/0; -- deliberate divide-by-zero to trigger an error
END TRY
BEGIN CATCH
    PRINT 'Logged: ' + ERROR_MESSAGE();
    THROW; -- bare THROW re-raises the exact original error
END CATCH;

The Error Functions Toolkit

All six are only valid inside a CATCH block — called anywhere else, they simply return NULL, since there's no error context to describe.

Nesting: A TRY/CATCH Inside a CATCH

BEGIN TRY
    UPDATE dbo.Product SET stock_qty = stock_qty - 10 WHERE product_id = 2;
END TRY
BEGIN CATCH
    BEGIN TRY
        INSERT INTO dbo.ErrorLog (error_message, logged_at) VALUES (ERROR_MESSAGE(), SYSDATETIME());
    END TRY
    BEGIN CATCH
        PRINT 'Even the error logging failed — this is genuinely bad, escalate.';
    END CATCH;
    THROW;
END CATCH;

This is a real, defensible pattern in production code: log the error to a table for later diagnosis, but wrap the logging itself in its own TRY/CATCH — you don't want a failure in your error-logging code to mask or replace the original error.