SQL Server Isolation Levels Comparison

Isolation levels control how one transaction is allowed to see changes made by other concurrent transactions.

Feature	READ UNCOMMITTED	READ COMMITTED (Default)	REPEATABLE READ	SNAPSHOT	SERIALIZABLE
Dirty Reads	✅ Possible	❌ No	❌ No	❌ No	❌ No
Non-Repeatable Reads	✅ Possible	✅ Possible	❌ No	❌ No	❌ No
Phantom Reads	✅ Possible	✅ Possible	✅ Possible	❌ No	❌ No
Reader Blocks Writer	❌ No	✅ Yes	✅ Yes	❌ No	✅ Yes
Writer Blocks Reader	❌ No	✅ Yes	✅ Yes	❌ No	✅ Yes
Uses Row Versioning	❌ No	❌ No	❌ No	✅ Yes	❌ No
TempDB Usage	❌ No	❌ No	❌ No	✅ Yes	❌ No
Concurrency	Highest	Good	Medium	Very High	Lowest
Data Consistency	Lowest	Good	Better	Very High	Highest