An isolation level determines the degree of isolation of data between concurrent transactions. The default SQL Server isolation level is Read Committed. A lower isolation level increases concurrency, but at the expense of data correctness. Conversely, a higher isolation level ensures that data is correct, but can affect concurrency negatively. The isolation level required by an application determines the locking behavior SQL Server uses.
SQL-92 defines the following isolation levels, all of which are supported by SQL Server:
- Read uncommitted (the lowest level where transactions are isolated only enough to ensure that physically corrupt data is not read).
- Read committed (SQL Server default level).
- Repeatable read.
- Serializable (the highest level, where transactions are completely isolated from one another).
| Isolation level | Dirty read | Nonrepeatable read | Phantom |
| Read uncommitted | Yes | Yes | Yes |
| Read committed | No | Yes | Yes |
| Repeatable read | No | No | Yes |
| Serializable | No | No | No |