An isolation level determines the degree of isolation of data between concurrent transactions. The default SQL Server isolation level is Read Committed. A lower isolation level increases concurrency, but at the expense of data correctness. Conversely, a higher isolation level ensures that data is correct, but can affect concurrency negatively. The isolation level required by an application determines the locking behavior SQL Server uses. SQL-92 defines the following isolation levels, all of which are supported by SQL Server:
Read uncommitted (the lowest level where transactions are isolated only enough to ensure that physically corrupt data is not read).
Read committed (SQL Server default level).
Repeatable read.
Serializable (the highest level, where transactions are completely isolated from one another).

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