SET TRANSACTION ISOLATION LEVEL (T-SQL)

Controls the default transaction locking behavior for all Microsoft® SQL Server™ SELECT statements issued by a connection.

Syntax

SET TRANSACTION ISOLATION LEVEL
    {
        READ COMMITTED
        | READ UNCOMMITTED
        | REPEATABLE READ
        | SERIALIZABLE
    }

Arguments

READ COMMITTED

Specifies that 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 nonrepeatable reads or phantom data. This option is the SQL Server default.

READ UNCOMMITTED

Implements dirty read, or isolation level 0 locking, which means that no shared locks are issued and no exclusive locks are honored. When this option is set, it is possible to read uncommitted or dirty data; values in the data can be changed and rows can appear or disappear in the data set before the end of the transaction. This option has the same effect as setting NOLOCK on all tables in all SELECT statements in a transaction. This is the least restrictive of the four isolation levels.

REPEATABLE READ

Locks are placed on all data that is used in a query, preventing other users from updating the data, but new phantom rows can be inserted into the data set by another user and are included in later reads in the current transaction. Because concurrency is lower than the default isolation level, use this option only when necessary.

SERIALIZABLE

Places a range lock on the data set, preventing other users from updating or inserting rows into the data set until the transaction is complete. This is the most restrictive of the four isolation levels. Because concurrency is lower, use this option only when necessary. This option has the same effect as setting HOLDLOCK on all tables in all SELECT statements in a transaction.

Remarks

Only one of the options can be set at a time, and it remains set for that connection until it is explicitly changed. This becomes the default behavior unless an optimization option is specified at the table level in the FROM clause of the statement.

The setting of SET TRANSACTION ISOLATION LEVEL is set at execute or run time and not at parse time.

Examples

This example sets the TRANSACTION ISOLATION LEVEL for the session. For each Transact-SQL statement that follows, SQL Server holds all of the shared locks until the end of the transaction.

SET TRANSACTION ISOLATION LEVEL REPEATABLE READ

GO

BEGIN TRANSACTION

SELECT * FROM publishers

SELECT * FROM authors

...

COMMIT TRANSACTION

See Also