How Can I Optimally Concatenate Strings in SQL Azure Without CLR Aggregate Functions?

SQL Azure string efficient connection solution

In SQL Azure, efficiently aggregating disparate string data into a single entity is critical. Due to the lack of CLR custom aggregate functions in SQL Azure, efficient alternatives need to be found.

Transact SQL Solution

An efficient way is to use WITH clause and recursive CTE (Common Table Expression). This method includes:

1. Use ROW_NUMBER() and PARTITION BY to number the rows within the partition and define the join order.
2. Use recursive CTE (Concatenated) iteratively to construct the concatenated string.
3. Filter the results to include only the final aggregated results (where NameNumber = NameCount).

Description

Please consider the following inquiry:

<code class="language-sql">;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;</code>

Example

For the following data:

<code>ID      Name
------- --------
1       Matt
1       Rocks
2       Stylus
3       Foo
3       Bar
3       Baz</code>

Returns the following aggregate results:

<code>ID      FullName
------- ---------------------
1       Matt, Rocks
2       Stylus
3       Foo, Bar, Baz</code>

Advantages

• For SQL Azure
• Relatively efficient, especially for smaller data sets
• Easy to customize for specific connection scenarios (e.g. sort order, separators)

Note: In order to avoid the length overflow of nvarchar data type, change the type of CAST to nvarchar(max). In addition, the sorting in the example results is slightly different from the original text. This is because the sorting basis of ROW_NUMBER() OVER (PARTITION BY ID ORDER BY Name) is the Name field, resulting in differences in the sorting results. If you need a specific sorting method, modify the ORDER BY clause.

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