Isolation levels in SQL Server

 Isolation levels in SQL Server define the degree to which one transaction must be isolated from resource or data modifications made by other transactions. SQL Server supports several isolation levels, each providing a different level of consistency, concurrency, and isolation. The isolation levels in SQL Server are defined by the SQL standard and include the following:

1. READ UNCOMMITTED:

• Description: Allows a transaction to read data that is being modified by another transaction without waiting for the other transaction to complete.
• Issues: Non-repeatable reads, dirty reads, and phantom reads are possible.

2. READ COMMITTED:

• Description: Ensures that a transaction reads only committed data. It prevents dirty reads but still allows non-repeatable reads and phantom reads.
• Issues: Non-repeatable reads and phantom reads are possible.

3. REPEATABLE READ:

• Description: Ensures that if a transaction reads a value, it will get the same value if it reads it again within the same transaction. It prevents dirty reads and non-repeatable reads but allows phantom reads.
• Issues: Phantom reads are possible.

4. SNAPSHOT:

• Description: Allows a transaction to read a version of data as it existed at the start of the transaction. This provides a consistent snapshot of the data for the duration of the transaction.
• Issues: Avoids dirty reads, non-repeatable reads, and phantom reads.

5. SERIALIZABLE:

• Description: Provides the highest level of isolation. It ensures that transactions are completely isolated from one another. It prevents dirty reads, non-repeatable reads, and phantom reads.
• Issues: Increased contention and potential for slower performance due to locks.

The isolation level can be set for a session using the SET TRANSACTION ISOLATION LEVEL statement. 

For example:

SET TRANSACTION ISOLATION LEVEL READ COMMITTED;

it's important to note that selecting a higher isolation level typically comes with an increased risk of performance issues, such as blocking and decreased concurrency. Developers and database administrators need to carefully choose the appropriate isolation level based on the requirements of the application and the specific trade-offs they are willing to make in terms of consistency and performance.