Microsoft® SQL Server 2012 Unleashed

The Need for Locking

In any multiuser database, there must be a consistent set of rules for making changes to the data. For a true transaction-processing database, the database management system (DBMS) is responsible for resolving potential conflicts between two different processes that are attempting to change the same piece of information at the same time. Such a situation cannot occur because the consistency of a transaction cannot be guaranteed. For example, if two users were to change the same data at approximately the same time, whose change would be propagated? Theoretically, the results would be unpredictable because the answer is dependent on whose transaction completed last. Because most applications try to avoid “unpredictability” ...