What are the transaction isolation levels in SQL Server?

In SQL Server, there are the following transaction isolation levels, which determine how transactions see each other's changes and prevent various concurrency issues:

1. Read Uncommitted — the lowest isolation level. Allows reading uncommitted data (dirty read). Can lead to reading inconsistent data.

2. Read Committed — the default level. Prevents dirty reads but allows non-repeatable reads, where data can change between two reads within the same transaction.

3. Repeatable Read — guarantees that data read within a transaction will not change until the transaction completes. Prevents non-repeatable reads but allows phantom reads (appearance of new rows).

4. Serializable — the strictest level. Prevents phantom reads, providing full transaction isolation as if transactions were executed sequentially.

5. Snapshot — uses row versioning (MVCC). The transaction sees data as of its start, preventing read locks and phantom reads, but may lead to update errors if data has changed.

Example of setting the isolation level:

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
BEGIN TRANSACTION;
-- operations
COMMIT;