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because row version does not make it possible to update more than 1 row atomically.

Yes it does. UPDATE foo = 'abc' WHERE (id, version) IN ((123, 1), (234, 2))

while true this is not what I mean.

if your goal is to transfer 10 dollars for account A to account B but only if account A balance is larger than 10 dollars.

You have to update account A to be 10 dollars less

and

you have to update account B to be 10 dollars more.

you can read both account a keep track of current version number for each.

then do

"UPDATE Accounts SET Balance = CASE AccountID WHEN 1 THEN Balance - 10 WHEN 2 THEN Balance + 10 END WHERE (AccountID, version) IN ((1,v1), (2,v2))"

But if using Read Uncommited isolation level I am not sure this UPDATE would actually lock both row until commit.

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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