In the real world, there's no perfect concurrency. If there are two mutual updates, only one can succeed updating the row at one moment (considering an ACID-compliant database) and the latter occurs afterwards.

However, TIMESTAMP column in MySQL (and others, probably) can store a fraction of a second up to 6 digits which seems to be sufficient for the most cases. But even if both updated_at fields are exactly equal, there's no harm since updates are always serialized on the same row.

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