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Editorial Team
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Asked: 2026-05-18T07:08:37+00:00

ok, I tried searching and have not found an answer to this – I

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ok, I tried searching and have not found an answer to this – I am curious how the ROLLBACK handles race conditions. For example:

If I have a table (CompanyAccount) which keeps track of how many credits an company has available for purchase (there is only one row in a database table per company) and there are potentially multiple users from the same company who can decrement the credits from the single company account, what happens in case of an error when a ROLLBACK occurs?

Example:

Assumptions: I have written the update properly to calculate the “Credit” new balance instead of guessing what the new credit balance is (i.e. we don’t try to tell the UPDATE statement what the new Credit balance/value is, we say take whatever is in the credit column and subtract my decrement value in the UPDATE statement)…

here is an example of how the update statement is written:

UPDATE dbo.CompanyAccount
SET Credit = Credit – @DecrementAmount
WHERE CompanyAccountId = @CompanyAccountId

If the “Credit” column has 10,000 credits. User A causes a decrement of 4,000 credits and User B causes a decrement of 1000 credits. For some reason a rollback is triggered during User A’s decrement (there are about a 1/2 dozen more tables with rows getting INSERTED during the TRANSACTION). If User A wins the race condition and the new balance is 6,000 (but not yet COMMIT’ed) what happens if User B’s decrement occurs before the rollback is applied? does the balance column go from 6,000 to 5,000 and then gets ROLLBACK to 10,000?

I am not too clear on how the ROLLBACK will handle this. Perhaps I am over-simplifying. Can someone please tell me if I misunderstand how ROLLBACK will work or if there are other risks I need to worry about for this style.

Thanks for your input.

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1 Answer

  1. Editorial Team

    In the example you have given there will be no problem.

    The first transaction will have an exclusive lock meaning the second one can not modify that row until after the first one has committed or rolled back. It will just have to wait (blocked) until the lock is released.

    It gets a bit more complicated if you have multiple statements. You should probably read up on different isolation levels and how they can allow or prevent such phenomena as “lost updates”.

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