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you still have to do

IF @@ROWCOUNT = 2 BEGIN COMMIT TRANSACTION; END ELSE BEGIN ROLLBACK TRANSACTION; END

to make sure both rows have been updated. so its a lot simpler to explicitly take a lock on the rows.

You're right. Locking the rows or using serializable isolation would be required to achieve an atomic operation if and only if both accounts are in the pristine state before it.

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