SQL Server supports DIFFERENT types of isolation level.

1. Read Uncommitted – Read Uncommitted is the lowest isolation level allow dirty reads. Here, CHANGES does by one TRANSACTION is visible to other transactions before committing.
2. Read Committed – Read Committed s isolation does not allow dirty read. Any data read by the transaction is 100% committed. Till the time transaction is updating the record, it holds exclusive locks on it.
3. Repeatable Read – Repeatable read is the most restrictive isolation level which holds locks on the table EVEN with read transactions. Table data cannot be modified from any other sessions until the transaction is completed.
4. Serializable – This is the Highest isolation level similar to repeatable read with the prevention of Phantom Read. It ensures transaction referencing same records must run in serially.
5. Snapshot – This also known as RCSI (Read Committed Snapshot Isolation). It’s similar to Serializable Isolation but the only difference is Snapshot does not hold a lock on the table during the transaction so that the table can be modified in other sessions. Snapshot isolation MAINTAINS versioning in Tempdb for old data called Row Versioning.