A transaction is a logical unit of work containing one or more steps, each of which must complete successfully in order for the transaction to commit to the database. If there are errors, then all of the data modifications are erased and the database is rolled back to its initial state at the start of the transaction.
BEGIN TRANSACTION INSERT INTO DeletedEmployees(EmployeeID, DateDeleted, User) (SELECT 123, GetDate(), CURRENT_USER); DELETE FROM Employees WHERE EmployeeID = 123; COMMIT TRANSACTION
When something fails in your transaction code and you want to undo it, you can rollback your transaction:
BEGIN TRY BEGIN TRANSACTION INSERT INTO Users(ID, Name, Age) VALUES(1, 'Bob', 24) DELETE FROM Users WHERE Name = 'Todd' COMMIT TRANSACTION END TRY BEGIN CATCH ROLLBACK TRANSACTION END CATCH