How to Efficiently Join Multiple Rows in SQL Server into a Single Delimited Field

In database management, especially when dealing with relational databases like SQL Server, you often find scenarios where you need to combine multiple rows into a single field for better readability and simpler processing. This blog post will address how to solve this problem using SQL Server functions, illustrating the desired outcome and effective solutions for different SQL Server versions.

The Problem Statement

Imagine you have two tables: Vehicles and Locations. Here’s a quick example of what these tables look like:

Vehicles Table

Locations Table

The goal is to pull data in a way that shows each vehicle’s name alongside a list of its associated locations. The desired result should look something like this:

While you could achieve this through complex queries and coding, we need a solution that simplifies the process.

Solution: Using SQL Server Functions

Method 1: Using FOR XML PATH

If you’re using SQL Server 2005 or later, one of the most effective methods to achieve this concatenation is the FOR XML PATH command. Here is how you can implement it:

SELECT [VehicleID],
       [Name],
       (STUFF((SELECT ', ' + [City] AS [text()]
                FROM [Location]
                WHERE (VehicleID = Vehicle.VehicleID)
                FOR XML PATH('')), 1, 2, '')) AS Locations
FROM [Vehicle]

Explanation:

• The inner SELECT statement retrieves the locations for each VehicleID and concatenates them into a single string.
• FOR XML PATH('') allows us to generate a single XML string from the multiple rows returned by the subquery.
• STUFF is used to remove the leading comma and space from the concatenated string.

Method 2: Using STRING_AGG (SQL Server 2017 and Later)

For those using SQL Server 2017 or newer, there is an even simpler way to achieve this using the STRING_AGG function. Here’s how it looks:

SELECT [VehicleID],
       [Name],
       (SELECT STRING_AGG([City], ', ')
        FROM [Location]
        WHERE VehicleID = V.VehicleID) AS Locations
FROM [Vehicle] V

Explanation:

• STRING_AGG takes two arguments: the column you want to aggregate, and the delimiter with which you want to separate the entries. It’s much clearer and efficient than the previous method.

Conclusion

Combining multiple rows into a single delimited field in SQL Server can significantly enhance your data’s readability and accessibility. Whether you’re on an older version and using FOR XML PATH or you’re able to use STRING_AGG in newer versions, SQL Server provides you with the tools you need to efficiently manage your data.

By understanding these techniques, you can simplify your SQL queries, making them easier to read, maintain, and modify in the future. Always choose the method that best fits your version of SQL Server and your specific requirements.

If you have any questions or need further clarification on these methods, feel free to ask in the comments below!