Isolation Level Basics

Randolph West describes the primary isolation levels in SQL Server:

There are four isolation levels in SQL Server (as quoted from SQL Server Books Online):

• Read uncommitted (the lowest level where transactions are isolated only enough to ensure that physically corrupt data is not read)
• Read committed (Database Engine default level)
• Repeatable read
• Serializable (the highest level, where transactions are completely isolated from one another)

Read on for a discussion of what these mean, as well as how optimistic versus pessimistic concurrency (in this case, Read Committed Snapshot Isolation versus Read Committed) comes into play.