Understanding and Managing SQL Server Compatibility Levels

Managing SQL Server databases effectively involves understanding and configuring various settings, and one crucial aspect is the Sql Server Compatibility Level. This setting dictates how the Database Engine behaves with respect to database compatibility with older versions of SQL Server. Setting the correct compatibility level is essential for ensuring your database applications function as expected after migrating to a newer SQL Server version or when dealing with databases created on different versions.

This article will guide you through understanding, viewing, and changing the compatibility level of your SQL Server databases, whether you are using SQL Server on-premises, Azure SQL Database, Azure SQL Managed Instance, or even Fabric SQL databases.

Why is SQL Server Compatibility Level Important?

The compatibility level in SQL Server is essentially a version switch. When you upgrade your SQL Server instance, databases do not automatically get upgraded to the latest database engine behavior. Instead, they retain their existing compatibility level, ensuring that existing applications continue to function without immediate code changes.

This is vital because new versions of SQL Server might introduce changes to query processing, data types, or features that could potentially break older applications. By maintaining the compatibility level of the original SQL Server version, you minimize the risk of application errors immediately after an upgrade.

However, running databases at older compatibility levels means you might not be taking full advantage of the performance improvements, new features, and optimizations available in the newer SQL Server version. Therefore, understanding how to manage compatibility levels is key to balancing application stability with leveraging the latest database engine capabilities.

Viewing the SQL Server Compatibility Level

There are two primary methods to check the compatibility level of a SQL Server database: using SQL Server Management Studio (SSMS) and using Transact-SQL (T-SQL) queries.

Using SQL Server Management Studio (SSMS)

SQL Server Management Studio provides a graphical interface to easily view the compatibility level:

Connect to your SQL Server instance using SSMS.
In Object Explorer, navigate to and expand the Databases node.
Right-click on the specific database you want to inspect (user or system database). Note that in Azure SQL Database, you cannot modify the compatibility level of system databases.
Select Properties from the context menu. This will open the Database Properties dialog box.
In the Database Properties dialog, choose Options from the Select a page pane on the left.
On the right side, you will find the Compatibility level dropdown list. The currently selected value indicates the compatibility level of the database.

This image shows the Compatibility level dropdown list within the Options page of a database’s Properties dialog in SQL Server Management Studio. It highlights where users can view and change the compatibility level setting.

Using Transact-SQL (T-SQL)

For a more programmatic approach, or when working with environments where a GUI is not available, you can use Transact-SQL to query the compatibility level:

Connect to your SQL Server instance using SSMS or Azure Data Studio.
Open a New Query window.
Execute the following T-SQL query, replacing YourDatabaseName with the actual name of your database:
```
USE YourDatabaseName;
GO
SELECT compatibility_level
FROM sys.databases
WHERE name = 'YourDatabaseName';
GO
```
This query selects the compatibility_level column from the sys.databases system catalog view for the specified database, returning the current compatibility level as a numerical value.

Changing the SQL Server Compatibility Level

Just as you can view the compatibility level through both SSMS and T-SQL, you can also modify it using both methods. Before changing the compatibility level, it is crucial to understand the implications for your applications. Refer to the official SQL Server documentation on ALTER DATABASE compatibility level for detailed information on potential breaking changes and behavior differences between compatibility levels.

Changing Compatibility Level Using SSMS

To change the compatibility level via SQL Server Management Studio:

Follow steps 1-5 from the “Viewing Compatibility Level Using SSMS” section to access the Compatibility level dropdown in the Database Properties dialog.
Select the desired compatibility level from the dropdown list. The available options will depend on the version of your SQL Server Database Engine. For example, 160 corresponds to SQL Server 2022 (16.x).
Click OK to save the changes.

Changing Compatibility Level Using Transact-SQL (T-SQL)

To change the compatibility level using T-SQL:

Connect to your SQL Server instance and open a New Query window.
Execute the following T-SQL command, replacing YourDatabaseName with the name of your database and XXX with the desired compatibility level (e.g., 160 for SQL Server 2022):
```
ALTER DATABASE YourDatabaseName
SET COMPATIBILITY_LEVEL = XXX;
GO
```
For instance, to set the AdventureWorks2022 database to SQL Server 2022 compatibility level:
```
ALTER DATABASE AdventureWorks2022
SET COMPATIBILITY_LEVEL = 160;
GO
```

Important Considerations When Changing Compatibility Levels

Testing is Key: Always thoroughly test your applications in a non-production environment after changing the compatibility level. This will help identify any potential issues before they impact your live systems.
Rollback Plan: Have a rollback plan in place. If you encounter problems after changing the compatibility level, you should be prepared to revert back to the previous setting.
Application Compatibility: Carefully review the SQL Server documentation for the specific compatibility level you are targeting to understand potential changes in behavior that might affect your applications.
Performance: While newer compatibility levels often bring performance improvements, ensure to monitor performance after the change. In some rare cases, specific queries might perform differently.
Fabric SQL Databases: Note that the compatibility level of Fabric SQL databases is always the latest version and cannot be manually changed.

Conclusion

Understanding and managing SQL Server compatibility levels is a critical aspect of database administration, especially during and after SQL Server upgrades. By correctly setting the compatibility level, you can ensure application stability while gradually adopting new SQL Server features and performance enhancements. Always prioritize testing and planning when making changes to compatibility levels to maintain a healthy and efficient database environment.