Isolation Level and its types in SQL Server

Isolation refers to the degree to which one transaction must be isolated from others. In SQL Server, there are four isolation levels, each providing a different degree of isolation. Here are the isolation levels and their examples:

Read Uncommitted: This isolation level allows transactions to read uncommitted data, which can result in dirty reads. In other words, a transaction can read data that has been modified but not yet committed by another transaction. This level is not recommended for most applications.

Read Committed: This isolation level guarantees that a transaction can only read committed data. A transaction will wait until another transaction has committed the data before it reads it. However, it is still possible to encounter non-repeatable reads and phantom reads.

Repeatable Read: This isolation level guarantees that a transaction will always see the same data when it reads it, even if the data has been modified by another transaction. However, it is still possible to encounter phantom reads, where new rows appear that were not present in the previous read.

Snapshot: When a transaction starts using the snapshot isolation level, a snapshot of the data is created, and the transaction reads the data from the snapshot instead of the current data. Any changes made by other transactions after the snapshot was created will not be visible to the current transaction until it commits. This allows the transaction to read and write data without blocking other transactions, and without the risk of dirty reads, non-repeatable reads, or phantom reads.

Serializable: This isolation level provides the highest level of isolation. It guarantees that transactions will not encounter dirty reads, non-repeatable reads, or phantom reads. However, this level can also lead to increased blocking and decreased concurrency.

To set the isolation level in SQL Server, you can use the SET TRANSACTION ISOLATION LEVEL statement. Here is an example of setting the isolation level to Read Committed:

SET TRANSACTION ISOLATION LEVEL READ COMMITTED

You can also set the isolation level for a specific transaction using the BEGIN TRANSACTION statement. Here is an example of setting the isolation level to Serializable for a transaction:

BEGIN TRANSACTION
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE

-- your transaction code here

COMMIT TRANSACTION

It's important to choose the appropriate isolation level for your application based on the balance between data consistency and performance. You can also use tools like the SQL Server Profiler to monitor the behavior of your transactions and choose the most appropriate isolation level for your application.

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