Minimum transaction isolation level to avoid “Lost Updates”

Question

With SQL Server\'s transaction isolation levels, you can avoid certain unwanted concurrency issues, like dirty reads and so forth.

The one I\'m interested in right n

Answer 1

As marked by Francis Rodgers, what you can rely on SQL Server implementation is that once a transaction updated some data, every isolation level always issue "update locks" over the data, and denying updates and writes from another transaction, whatever it's isolation level it is. You can be sure this kind of lost updates are covered.

However, if the situation is that a transaction reads some data (with an isolation level different than Repeatable Read), then another transaction is able to change this data and commits it's change, and if the first transaction then updates the same data but this time, based on the internal copy that he made, the management system cannot do anything for saving it.

Your answer in that scenario is either use Repeatable Read in the first transaction, or maybe use some read lock from the first transaction over the data (I don't really know about that in a confident way. I just know of the existence of this locks and that you can use them. Maybe this will help anyone who's interested in this approach Microsoft Designing Transactions and Optimizing Locking).