How to Efficiently Concatenate Strings from Multiple Rows in SQL Server Without CLR Functions?

High-Performance String Concatenation in SQL Server: Bypassing CLR Limitations

Aggregating strings from multiple rows into a single string is a frequent SQL Server task. While methods like COALESCE and FOR XML PATH exist, they often fall short in terms of efficiency. This is particularly true in SQL Azure, which lacks support for CLR functions, eliminating a common high-performance solution.

Effective Alternatives: Recursive CTE Approach

A robust and efficient alternative leverages a recursive Common Table Expression (CTE). This technique involves these key steps:

1. Row Numbering and Partitioning: Assigns a unique row number to each name within each distinct ID partition.
2. Recursive String Aggregation: Recursively concatenates strings by joining partitions based on sequential row numbers.
3. Result Filtering: Selects only the final concatenated string for each partition.

This method guarantees consistent results through explicit grouping and sorting.

Practical Implementation:

Here's a sample implementation using a recursive CTE:

WITH Partitioned AS (
    SELECT 
        ID,
        Name,
        ROW_NUMBER() OVER (PARTITION BY ID ORDER BY Name) AS NameNumber,
        COUNT(*) OVER (PARTITION BY ID) AS NameCount
    FROM dbo.SourceTable
),
Concatenated AS (
    SELECT 
        ID, 
        CAST(Name AS nvarchar(max)) AS FullName, 
        Name, 
        NameNumber, 
        NameCount 
    FROM Partitioned 
    WHERE NameNumber = 1
    UNION ALL
    SELECT 
        P.ID, 
        CAST(C.FullName + ', ' + P.Name AS nvarchar(max)), 
        P.Name, 
        P.NameNumber, 
        P.NameCount
    FROM Partitioned AS P
        INNER JOIN Concatenated AS C 
            ON P.ID = C.ID 
            AND P.NameNumber = C.NameNumber + 1
)
SELECT 
    ID,
    FullName
FROM Concatenated
WHERE NameNumber = NameCount;

This recursive CTE provides a scalable and efficient solution for string concatenation in SQL Server, especially in environments where CLR functions are unavailable. Note the use of nvarchar(max) to handle potentially large concatenated strings.

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