How Can I Efficiently Concatenate Strings from Multiple Rows in SQL Azure?

Optimizing String Aggregation in SQL Azure

Efficiently combining strings from multiple rows into a single row is a frequent data manipulation task. While some aggregation methods prove inadequate, optimal solutions exist to overcome this challenge.

The Best Approach for SQL Azure

SQL Azure's lack of CLR-defined aggregate functions necessitates alternative strategies. The following Transact-SQL approach provides an efficient solution:

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;

Detailed Explanation:

This solution employs a three-part process:

1. Row Numbering: Assigns a unique number to each row within each ID partition, ordered alphabetically by Name, using ROW_NUMBER() OVER (PARTITION BY ID ORDER BY Name).
2. Recursive Concatenation: A recursive Common Table Expression (CTE) iteratively appends Name values based on the assigned row numbers.
3. Final Selection: Filters the results to retain only the rows with the highest NameNumber within each partition, yielding a single concatenated string per ID.

Note: This query assumes grouping by ID and ascending alphabetical order of strings. Adaptations may be needed depending on your specific data structure and requirements. The nvarchar(max) cast ensures sufficient string length for large concatenated results.

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