In SQL Server, isolation refers to the level of locking and concurrency control used when multiple transactions access the same data simultaneously. There are several isolation levels in SQL Server, each with its own level of locking and concurrency control.

The most common isolation levels are:

READ UNCOMMITTED: This is the lowest isolation level. It allows a transaction to read data that is being modified by other transactions, even if those transactions have not yet been committed. This can lead to dirty reads (reading data that is later rolled back) and non-repeatable reads (reading data that is later modified by another transaction).

READ COMMITTED: This is the default isolation level in SQL Server. It ensures that a transaction can only read data that has been committed by other transactions. This eliminates the possibility of dirty reads, but non-repeatable reads are still possible.

REPEATABLE READ: This isolation level ensures that a transaction can only read data that has been committed by other transactions, and that data cannot be modified by other transactions until the current transaction is committed. This eliminates the possibility of dirty reads and non-repeatable reads, but can lead to increased contention and reduced concurrency.

SERIALIZABLE: This is the highest isolation level. It ensures that a transaction has a completely isolated view of the data, and that no other transaction can modify the data while the current transaction is running. This eliminates the possibility of dirty reads, non-repeatable reads, and phantom reads (reading data that is later inserted by another transaction), but can lead to increased contention and reduced concurrency.

Example

Here's an example of how to set the isolation level to READ COMMITTED in SQL Server:

SET TRANSACTION ISOLATION LEVEL READ COMMITTED;

This command sets the isolation level for the current transaction to READ COMMITTED. This means that each query in the transaction will only see committed data and will not be blocked by uncommitted data.

Another example of isolation in SQL Server is the use of locking hints. Locking hints allow you to control the level of locking used when accessing data in SQL Server. Here's an example:

SELECT * FROM orders WITH (UPDLOCK, HOLDLOCK)
WHERE order_id = 12345;

This command uses two locking hints, UPDLOCK and HOLDLOCK, to lock the row with order_id 12345. UPDLOCK requests an update lock, which prevents other transactions from updating the row while the current transaction is accessing it. HOLDLOCK requests a shared lock, which prevents other transactions from accessing the row while the current transaction is accessing it.

It is important to note that isolation level does not affect the order in which transactions are executed, it only affects what data a transaction can see and modify.