A statement can use data only if it will be in a consistent state throughout the transaction. If another transaction modifies data after the start of the current transaction, the data is not visible to the current transaction. The current transaction works with a snapshot of the data as it existed at the beginning of that transaction. Snapshot transactions do not request locks when reading data, nor do they block other transactions from writing data. In addition, other transactions writing data do not block the current transaction for reading data. As with the Serializable isolation level, the Snapshot level prevents dirty reads, nonrepeatable reads and phantom reads.