Why should you care?

Creating an order and deducting stock must not leave half-done work if the second step fails.

See it live — copy this example

Run in SQL Server Management Studio (SSMS) or Azure Data Studio.

USE DataVerse;
BEGIN TRY
    BEGIN TRAN;
    UPDATE dbo.Accounts SET Balance = Balance - 100 WHERE AccountId = 1;
    UPDATE dbo.Accounts SET Balance = Balance + 100 WHERE AccountId = 2;
    COMMIT;
END TRY
BEGIN CATCH
    IF @@TRANCOUNT > 0 ROLLBACK;
    THROW;
END CATCH;

What happened?

• Both balance updates succeed together or neither keeps changes.
• CATCH rolls back if anything throws mid-way.

Practice next

1. Seed two Accounts with known balances.
2. Run the transfer transaction.
3. Force an error after the first UPDATE and confirm rollback.
4. Add an INSERT into a TransferAudit table inside the tran.
5. Nest a savepoint (covered later concepts).

Remember

Transactions make multi-step changes atomic. COMMIT finishes; ROLLBACK undoes. Always handle errors with rollback.

Checkout must be atomic

DataVerse checkout writes order + decreases stock in one tran.

Outcome: No order without stock movement, and the reverse.