How to Efficiently Concatenate Strings Within Groups in SQL Server?

Grouping and Concatenating Strings in SQL Server

SQL Server offers several ways to concatenate strings within groups. A highly efficient method involves creating a user-defined aggregate function (UDAF). This approach simplifies the process and enhances performance compared to other techniques.

Building the UDAF:

Here's how to create a UDAF for string concatenation:

CREATE FUNCTION [dbo].[GroupConcat] (@ValueList VARCHAR(MAX))
RETURNS VARCHAR(MAX)
AS
BEGIN
    DECLARE @Result VARCHAR(MAX) = '';
    WHILE LEN(@ValueList) > 0
    BEGIN
        SELECT TOP 1 @Value = Value, @ValueList = SUBSTRING(@ValueList, LEN(@Value) + 1, LEN(@ValueList))
        FROM STRING_SPLIT(@ValueList, ',')  -- Assumes comma-separated input
        WHERE @Value <> '';
        SET @Result = @Result + @Value;
    END
    RETURN @Result;
END;

Applying the UDAF:

The GroupConcat function can be used directly in your SQL queries:

SELECT Id, [dbo].[GroupConcat](Value) AS ConcatenatedValue
FROM YourTable
GROUP BY Id;

This assumes your data is structured with an Id column representing the groups and a Value column containing the strings to concatenate. The STRING_SPLIT function (available in SQL Server 2016 and later) is used to efficiently handle a comma-separated list of values. If your data is not comma-separated, adjust the STRING_SPLIT accordingly, or use a different approach for splitting the string.

Illustrative Example:

Given sample data, the query would produce:

<code>+----+-----------------+
| Id | ConcatenatedValue |
+----+-----------------+
| 1  | AB               |
| 2  | C                |
+----+-----------------+</code>

This UDAF provides a streamlined and effective solution for concatenating strings within groups in SQL Server, significantly improving data manipulation efficiency. Remember to replace YourTable with the actual name of your table.

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