How Can I Efficiently Concatenate Strings from Multiple Rows in SQL Azure Without CLR Functions?

Beyond COALESCE and FOR XML: Efficient String Aggregation in SQL Azure

Many developers seeking efficient string concatenation from multiple SQL rows encounter limitations with standard functions like COALESCE and FOR XML in SQL Azure, especially when CLR functions aren't available. This article presents a powerful Transact-SQL solution using Common Table Expressions (CTEs) for robust and efficient string aggregation.

The Solution: Recursive CTEs for Sequential Concatenation

Our approach leverages two CTEs:

1. Partitioned CTE: This assigns row numbers to each entry based on the ID column, ordering alphabetically by the Name column. This crucial step groups rows with the same ID and ensures consistent concatenation order.

2. Concatenated CTE (Recursive): This CTE iteratively builds the concatenated string. It recursively appends names to a FullName column, accumulating the final result.

The main query then selects only the rows with the highest row number for each ID, yielding the complete aggregated string for each group.

Detailed Breakdown and Customization Options

The method comprises three core stages:

1. Row Partitioning and Numbering: This establishes the grouping and ordering necessary for accurate concatenation.
2. Recursive String Accumulation: The recursive CTE efficiently builds the aggregated string within the FullName column.
3. Result Filtering: The final query selects only the complete concatenated strings, one for each unique ID.

This technique offers flexibility. You can adjust the grouping (ID in this example) and sorting criteria (alphabetical order of Name here) to fit your specific data structure and requirements. Consistent results depend on defining both grouping and sorting parameters.

Illustrative Example and Output

Let's use this sample data:

<code class="language-sql">INSERT dbo.SourceTable (ID, Name)
VALUES 
(1, 'Matt'),
(1, 'Rocks'),
(2, 'Stylus'),
(3, 'Foo'),
(3, 'Bar'),
(3, 'Baz')</code>

Executing the query will produce:

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

This clearly demonstrates the effective concatenation of strings across multiple rows, providing a reliable alternative to CLR functions for SQL Azure string aggregation tasks.

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