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.
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