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At a high level of isolation, the record added by client A will not appear in the sum calculated by client B; at a lower level, it will.

Unrepeatable reads: An unrepeatable read is said to occur if a transaction sees different result sets from the same query while it is in progress.My SQL makes it possible to control the isolation level via the special TRANSACTION ISOLATION LEVEL variable.Consider the following examples, which illustrate: In order to illustrate, let’s consider a simple example.I’m not going to get into the details of each of these problems at this stage, you should take a look at the links at the end of this article for more information.Suffice to say that they do exist, and in large RDBMS, steps need to be taken to prevent them from corrupting the system with misleading data while simultaneously not causing too great an impact on performance.

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