Isolation Levels — degree of transaction isolation from concurrent operations.
Four Levels:
1. Read Uncommitted:
- Can read uncommitted changes
- Dirty reads possible
2. Read Committed:
- Reads only committed data
- Non-repeatable reads possible
3. Repeatable Read:
- Same query returns same results
- Phantom reads possible
4. Serializable:
- Full isolation
- Phantom reads prevented
- Lowest concurrency
Common Problems:
Trade-off: Higher isolation = lower concurrency.