You use Microsoft .NET Framework 4 to develop an application that connects to a Microsoft SQL
Server 2008 database. You need to prevent dirty or phantom reads. Which IsolationLevel should you
use?
A.
Snapshot
B.
Serializable
C.
ReadUncommited
D.
ReadCommitted
Explanation:
Unspecified A different isolation level than the one specified is being used, but the level cannot be
determined.
When using OdbcTransaction, if you do not set IsolationLevel or you set IsolationLevel to
Unspecified, the transaction executes according to the isolation level that is determined by the
driver that is being used.
Chaos The pending changes from more highly isolated transactions cannot be overwritten.
ReadUncommitted A dirty read is possible, meaning that no shared locks are issued and no exclusive
locks are honored.
ReadCommitted Shared locks are held while the data is being read to avoid dirty reads, but the data
can be changed before the end of the transaction, resulting in non-repeatable reads or phantom
data.
RepeatableRead Locks are placed on all data that is used in a query, preventing other users from
updating the data.
Prevents non-repeatable reads but phantom rows are still possible.
Serializable A range lock is placed on the DataSet, preventing other users from updating or inserting
rows into the dataset until the transaction is complete.
Snapshot Reduces blocking by storing a version of data that one application can read while another
is modifying the same data.
Indicates that from one transaction you cannot see changes made in other transactions, even if you
requery.
IsolationLevel Enumeration
(http://msdn.microsoft.com/en-us/library/system.data.isolationlevel.aspx)
Isolation Levels in Database Engine(http://msdn.microsoft.com/en-us/library/ms189122.aspx)
SET TRANSACTION ISOLATION LEVEL (Transact-SQL)
(http://msdn.microsoft.com/ru-ru/library/ms173763.aspx)