I was recently asked by a friend, how can we control the level of locking in SQL Server when executing SELECT, UPDATE, INSERT and DELETE statements.
The answer to this question is by using locking hints. Locking hints direct SQL Server to the type of locks to be used. Even though the SQL Server Query Optimizer automatically determines the best locking option when executing a statement of the above kinds, there are cases where a DBA/Database Developer might needs to explicitly control the level of locking.
Available Locking Hints in SQL Server
The available locking hints in SQL Server are the following:
HOLDLOCK
Applies to: SELECT, UPDATE, INSERT, DELETE
NOLOCK
Applies to: SELECT
PAGLOCK
Applies to: SELECT, UPDATE, INSERT, DELETE
READCOMMITTED
Applies to: SELECT, UPDATE, INSERT, DELETE
READPAST
Applies to: SELECT, UPDATE, DELETE
READUNCOMMITTED
Applies to: SELECT
REPEATABLEREAD
Applies to: SELECT, UPDATE, INSERT, DELETE
ROWLOCK
Applies to: SELECT, UPDATE, INSERT, DELETE
SERIALIZABLE
Applies to: SELECT, UPDATE, INSERT, DELETE
TABLOCK
Applies to: SELECT, UPDATE, INSERT, DELETE
TABLOCKX
Applies to: SELECT, UPDATE, INSERT, DELETE
UPDLOCK
Applies to: SELECT, UPDATE, INSERT, DELETE
XLOCK
Applies to: SELECT, UPDATE, INSERT, DELETE
The syntax for using locking hints is very simple. You only have to add the expression with (LOCKING_HINT) just right after you are referencing a database table name (or after a table alias) in your query.
Example of Using Locking Hints in SQL Server
An example of using the NOLOCK locking hint within a SELECT statement is the following:
SELECT column FROM table with (NOLOCK)
The following MSDN Library link fully describes the above locking hints.
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Artemakis Artemiou, a distinguished Senior Database and Software Architect, brings over 20 years of expertise to the IT industry. A Certified Database, Cloud, and AI professional, he earned the Microsoft Data Platform MVP title for nine consecutive years (2009-2018). As the founder of SQLNetHub and GnoelixiAI Hub, Artemakis is dedicated to sharing his knowledge and democratizing education on various fields such as: Databases, Cloud, AI, and Software Development. His commitment to simplicity and knowledge sharing defines his impactful presence in the tech community.