What Are Isolation Levels in SQL Server?

Isolation levels in SQL Server determine how one transaction is isolated from others in a multi-user environment. In simpler terms, they control how and when the effects of one transaction become visible to others.

The primary goal is to maintain data integrity and avoid problems such as:

  • Dirty reads: Reading uncommitted data

  • Non-repeatable reads: Data changes between reads

  • Phantom reads: New records appear between queries

Why Do We Need Isolation Levels?

Proper isolation levels help you:

  • Ensure data consistency and transactional integrity

  • Avoid data anomalies (dirty reads, lost updates, etc.)

  • Control lock durations to balance performance vs. consistency

  • Fine-tune concurrency control based on business requirements

Best practice in 2025: Use appropriate isolation levels depending on your workload (OLTP vs. OLAP) to reduce deadlocks and improve query performance.

Types of Isolation Levels in SQL Server

SQL Server supports the following isolation levels, each with different locking behavior:

Isolation Level Prevents Allows Locks Held
READ UNCOMMITTED Nothing Dirty, Phantom No shared locks
READ COMMITTED Dirty Reads Phantom, NR Reads Shared locks on read
REPEATABLE READ Dirty, NR Reads Phantom Shared locks till commit
SERIALIZABLE Dirty, NR, Phantom Nothing Range/table locks
SNAPSHOT Dirty, NR, Phantom Nothing Uses versioning (TempDB)

Real-Time Examples of SQL Server Isolation Levels

a. READ COMMITTED (Default)

Behavior: Reads only committed data. Waits if a row is locked by another transaction.

sql
-- Session 1
BEGIN TRAN
UPDATE Emp SET Salary = 50000 WHERE EmpID = 1
-- Do not commit or rollback— Session 2
SELECT * FROM Emp WHERE EmpID = 1
— This will wait until Session 1 commits or rollbacks

 b. READ UNCOMMITTED

Behavior: Reads uncommitted (dirty) data. No locks, no wait time.

sql

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED

— Session 2
SELECT * FROM Emp WHERE EmpID = 1
— Reads dirty data from Session 1

c. REPEATABLE READ

Behavior: Prevents changes to rows read by a transaction. No new updates or deletes allowed.

sql
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ
BEGIN TRAN
SELECT * FROM Emp WHERE EmpID = 1
-- Session 2 cannot update/delete EmpID 1 until Session 1 commits

 d. SERIALIZABLE

Behavior: Strongest isolation. Prevents phantom reads using range locks.

sql
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
BEGIN TRAN
SELECT * FROM Emp WHERE EmpID BETWEEN 1 AND 3
-- Locks the range; new inserts in this range are blocked

e. SNAPSHOT

Behavior: Reads a consistent snapshot of data using TempDB versioning. No locks held.

sql

ALTER DATABASE YourDB SET ALLOW_SNAPSHOT_ISOLATION ON

SET TRANSACTION ISOLATION LEVEL SNAPSHOT
BEGIN TRAN
SELECT * FROM Emp WHERE EmpID = 1
— Sees old value even if another transaction modifies it

Which Isolation Level Should You Use in 2025?

Use Case Recommended Level
High concurrency, low risk READ UNCOMMITTED
Default general use READ COMMITTED
Preventing lost updates REPEATABLE READ
Full consistency (range queries) SERIALIZABLE
Performance + consistency SNAPSHOT (with TempDB)

Final Thoughts

Understanding and configuring the right isolation level in SQL Server is crucial for building robust, high-performance applications. With technologies advancing and workloads scaling in 2025, it’s more important than ever to optimize transaction control for speed, scalability, and accuracy.