How to Efficiently Combine Multiple Subquery Rows into a Single Delimited Field in SQL Server?

Streamlining SQL Server Data Manipulation: Combining Subquery Rows into a Single Delimited Field

Complex data manipulation tasks often involve writing cumbersome code, such as cursor-based solutions. A common challenge is combining multiple subquery rows into a single field with a delimiter. This article presents efficient alternatives to lengthy cursor implementations.

Consider two tables, "Vehicles" and "Locations," where the goal is to consolidate city names associated with each vehicle into a single "Locations" column, comma-separated. The traditional cursor approach requires extensive coding.

SQL Server 2005 and later:

A more elegant solution utilizes the FOR XML PATH command. This generates XML from the "Locations" table, separating city names with commas, and then converts the XML back to a text string using the STUFF function:

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

SQL Server 2017 and later:

For improved performance and simplicity, SQL Server 2017 and later versions offer the STRING_AGG function. This function directly aggregates values into a single string, accepting a separator as the second parameter:

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

Both methods effectively combine multiple subquery rows into a single delimited field, offering significant improvements over cursor-based approaches, resulting in cleaner, more efficient code and reduced development time. Choose the method appropriate for your SQL Server version.

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