Explain SQL Server Transaction Isolation Levels with practical use-cases.

Isolation levels include:

READ UNCOMMITTED: Allows dirty reads; used in analytics.

READ COMMITTED: Default; prevents dirty reads.

RCSI: Uses version store; avoids blocking.

REPEATABLE READ: Prevents non-repeatable reads.

SERIALIZABLE: Highest isolation; heavy locking.

SNAPSHOT: Uses row versioning; avoids shared locks.