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I am an MCSE in Data Management and Analytics, specializing in MS SQL Server, and an MCP in Azure. With over 19+ years of experience in the IT industry, I bring expertise in data management, Azure Cloud, Data Center Migration, Infrastructure Architecture planning, as well as Virtualization and automation. I have a deep passion for driving innovation through infrastructure automation, particularly using Terraform for efficient provisioning. If you're looking for guidance on automating your infrastructure or have questions about Azure, SQL Server, or cloud migration, feel free to reach out. I often write to capture my own experiences and insights for future reference, but I hope that sharing these experiences through my blog will help others on their journey as well. Thank you for reading!

Changing Database Compatibility Level in SQL Server

 

Changing Database Compatibility Level in SQL Server: A Step-by-Step Guide for New DBAs

As a new DBA, you'll often need to adjust your database settings to optimize performance or maintain compatibility with applications built for older SQL Server versions. One common task is changing the compatibility level of a database. In this blog post, we’ll walk you through the process of changing the compatibility level of a database named sampledatabase using a simple SQL command, and we’ll also review the various compatibility levels available in SQL Server to date.

Table of Contents

  1. Introduction
  2. What is Database Compatibility Level?
  3. Different Compatibility Levels in SQL Server
  4. Understanding the Command
  5. Step-by-Step Implementation
  6. Best Practices and Considerations
  7. Conclusion

Introduction

SQL Server compatibility levels allow you to control certain database behaviors to mimic older SQL Server versions. This is particularly useful when upgrading databases or ensuring legacy applications continue to work as expected. Changing the compatibility level can affect query processing, available features, and overall performance. In this guide, we will set the compatibility level of sampledatabase to 100 (SQL Server 2008 compatibility) using the ALTER DATABASE command and provide an overview of the different compatibility levels available in SQL Server.

What is Database Compatibility Level?

The compatibility level of a SQL Server database defines how certain features and behaviors operate within the database engine. By setting a specific compatibility level, you can control:

  • Query Optimization: Different versions of SQL Server have different query optimizers.
  • Feature Set: Newer features might be disabled or behave differently when using an older compatibility level.
  • Legacy Support: Applications that were developed with older SQL Server versions in mind can continue to work without modification.

Different Compatibility Levels in SQL Server

SQL Server has evolved over the years, and each version introduced its own compatibility level. Here are the common compatibility levels available:

  • 80: SQL Server 2000
    (For legacy systems, rarely used today.)

  • 90: SQL Server 2005
    (Introduced enhancements in query processing and management.)

  • 100: SQL Server 2008 / 2008 R2
    (Improved performance and additional T-SQL enhancements.)

  • 110: SQL Server 2012
    (Introduced new T-SQL features and performance improvements.)

  • 120: SQL Server 2014
    (Enhanced performance, improved indexing, and new security features.)

  • 130: SQL Server 2016
    (Brought in support for new analytics and in-memory OLTP enhancements.)

  • 140: SQL Server 2017
    (Further optimizations and support for Linux environments.)

  • 150: SQL Server 2019
    (Advanced performance enhancements, big data clusters, and improved scalability.)

  • 160: SQL Server 2022
    (Latest enhancements in performance, security, and cloud integration.)

Each compatibility level allows your database to emulate the behavior of the corresponding version of SQL Server, which can be critical when migrating older applications or when you want to test specific behaviors.

Understanding the Command

The command used to change the compatibility level is as follows:

USE [master];
GO
ALTER DATABASE [sampledatabase] SET COMPATIBILITY_LEVEL = 100;
GO

Breaking Down the Command:

  • USE [master];
    Switches the context to the master database, which is often the safest context for executing system-level commands.

  • GO
    Acts as a batch separator in SQL Server Management Studio (SSMS).

  • ALTER DATABASE [sampledatabase] SET COMPATIBILITY_LEVEL = 100;
    This command instructs SQL Server to set the compatibility level of sampledatabase to 100, which aligns with the behavior of SQL Server 2008.

  • GO
    Marks the end of the command batch.

Step-by-Step Implementation

Step 1: Open SQL Server Management Studio (SSMS)

Launch SSMS and connect to your SQL Server instance where sampledatabase resides.

Step 2: Open a New Query Window

Once connected, open a new query window and ensure you have the necessary permissions to alter database settings.

Step 3: Switch to the Master Database

Although not strictly necessary for altering a database's compatibility level, it’s a common best practice:

USE [master];
GO

Step 4: Change the Compatibility Level

Enter the following command to set the compatibility level of sampledatabase to 100:

ALTER DATABASE [sampledatabase] SET COMPATIBILITY_LEVEL = 100;
GO

Step 5: Execute the Commands

Run the batch of commands. SQL Server will update the compatibility level for sampledatabase. To verify the change, you can check the database properties in SSMS or query the system views.

Best Practices and Considerations

Test Changes in a Non-Production Environment

Before applying any changes to production databases, test them in a development or staging environment. This minimizes the risk of unexpected behavior.

Understand the Impact

Changing the compatibility level can affect:

  • Query optimization and execution plans.
  • The availability and behavior of certain T-SQL features.
  • Overall performance and behavior of the database engine.

Make sure to review Microsoft’s documentation for the compatibility level you plan to set.

Backup Your Database

Always create a backup of your database before making configuration changes. This allows you to restore the database if any issues arise after the change.

Document the Changes

Keep a record of any changes made to database settings, including the compatibility level. Document the reason for the change, the date, and any observations regarding system performance or application behavior.

Conclusion

Changing the compatibility level of a SQL Server database is a straightforward process that can have significant impacts on how your database functions. By setting the compatibility level of sampledatabase to 100, you’re instructing SQL Server to emulate the behavior of SQL Server 2008, which may be necessary for legacy applications or specific performance considerations.

In this guide, we explained:

  • What a compatibility level is and why it matters.
  • The different compatibility levels available in SQL Server from version 2000 through SQL Server 2022.
  • The step-by-step process of changing a database’s compatibility level using the ALTER DATABASE command.
  • Best practices to ensure your changes are safe, well-documented, and reversible if necessary.

As you continue your journey as a DBA, understanding how to manage and adjust the compatibility levels of your databases will be a vital skill. If you have any questions or need further clarification on this process, please feel free to leave a comment below.

Happy database managing!

Keep exploring and learning—each step you take builds your expertise as a confident and effective SQL Server DBA!

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