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Join Date: Nov 2008
Re: How to lock tables in SQL Server
The following kind of locks are available in SQL Server which you can apply with SELECT, INSERT, UPDATE, and DELETE statements:
- HOLDLOCK: Hold a shared lock until completion of the transaction instead of releasing the lock as soon as the required table, row, or data page is no longer required. HOLDLOCK is equivalent to SERIALIZABLE.
- NOLOCK: Do not issue shared locks and do not honor exclusive locks. When this option is in effect, it is possible to read an uncommitted transaction or a set of pages that are rolled back in the middle of a read. Dirty reads are possible. Only applies to the SELECT statement.
- PAGLOCK: Use page locks where a single table lock would usually be taken.
- READCOMMITTED: Perform a scan with the same locking semantics as a transaction running at the READ COMMITTED isolation level. By default, SQL Server 2000 operates at this isolation level.
- READPAST: Skip locked rows. This option causes a transaction to skip rows locked by other transactions that would ordinarily appear in the result set, rather than block the transaction waiting for the other transactions to release their locks on these rows. The READPAST lock hint applies only to transactions operating at READ COMMITTED isolation and will read only past row-level locks. Applies only to the SELECT statement.
- READUNCOMMITTED: Equivalent to NOLOCK.
- REPEATABLEREAD: Perform a scan with the same locking semantics as a transaction running at the REPEATABLE READ isolation level.
- ROWLOCK: Use row-level locks instead of the coarser-grained page- and table-level locks.
- SERIALIZABLE: Perform a scan with the same locking semantics as a transaction running at the SERIALIZABLE isolation level. Equivalent to HOLDLOCK.
- TABLOCK: Use a table lock instead of the finer-grained row- or page-level locks. SQL Server holds this lock until the end of the statement. However, if you also specify HOLDLOCK, the lock is held until the end of the transaction.
- TABLOCKX: Use an exclusive lock on a table. This lock prevents others from reading or updating the table and is held until the end of the statement or transaction.
- UPDLOCK: Use update locks instead of shared locks while reading a table, and hold locks until the end of the statement or transaction. UPDLOCK has the advantage of allowing you to read data (without blocking other readers) and update it later with the assurance that the data has not changed since you last read it.
- XLOCK: Use an exclusive lock that will be held until the end of the transaction on all data processed by the statement. This lock can be specified with either PAGLOCK or TABLOCK, in which case the exclusive lock applies to the appropriate level of granularity.