The OPENQUERY function in SQL Server is used to execute a pass-through query on a linked server. This allows you to run a SQL query against an external data source without needing to fetch all the tables or data into SQL Server first. To use OPENQUERY, you must have a linked server defined in your SQL Server.
Syntax
The basic syntax for using OPENQUERY is:
OPENQUERY(linked_server_name, 'query')
- linked_server_name: The name of the linked server as defined in SQL Server.
- query: A valid SQL statement that you want to execute on the linked server. This should be enclosed in single quotes.
Steps to Use OPENQUERY
1. Ensure Linked Server is Set Up: If you haven't already done so, create the linked server. You can create a linked server using SQL Server Management Studio (SSMS) or with the sp_addlinkedserver stored procedure.
2. Formulate Your Query: Write the SQL query that you want to execute on the linked server. Ensure that the query is compatible with the data source you are querying.
3. Execute the OPENQUERY: Use the OPENQUERY function in your SQL command.
Example
Assuming you have a linked server named MyLinkedServer, and you want to execute a query to select data from a table called Orders in that server, your code would look like this:
SELECT *
FROM OPENQUERY(MyLinkedServer, 'SELECT * FROM Orders WHERE OrderDate >= ''2023-01-01''')
Important Notes
1. Use of Single Quotes: In the SQL statement passed to OPENQUERY, you should use single quotes. If you need to include a single quote within the string (for example, in a date), you can escape it by doubling it (i.e., '').
2. Performance Considerations: Since OPENQUERY runs the SQL query on the linked server directly, it can be more efficient for complex queries when compared to fetching data back to SQL Server.
3. Permissions: Ensure that the account running this query has the necessary permissions on both the SQL Server and the linked server.
4. No Parameterization: You cannot directly pass parameters to OPENQUERY. All values must be explicitly determined in the string.
Conclusion
Using OPENQUERY can effectively facilitate the execution of queries against linked servers, but be mindful of the limitations and requirements. With proper setup and usage, it can become a powerful part of your SQL Server data operations when integrating external data sources.