Few important things about isolation level

Read uncommitted

When its used, SQL Server not issue shared locks while reading data. So, you can read an uncommitted transaction that might get rolled back later. This isolation level is also called dirty read. This is the lowest isolation level. It ensures only that a physically corrupt data will not be read.

Read committed

This is the default isolation level in SQL Server. When its used, SQL Server will

use shared locks while reading data. It ensures that a physically corrupt data will notbe read and will never read data that another application has changed and not yet committed,but it does not ensure that the data will not be changed before the end of the transaction.

Repeatable read

When its used, the dirty reads and nonrepeatable reads cannot occur.

It means that locks will be placed on all data that is used in a query,

and another transactions cannot update the data.

Nonrepeatable read

When a transaction reads the same row more than one time, and between the

two (or more) reads, a separate transaction modifies that row. Because the

row was modified between reads within the same transaction, each read

produces different values, which introduces inconsistency.