How can SQL Server's STUFF function and FOR XML PATH clause be used together to concatenate strings within a group?

SQL Server: Combining STUFF and FOR XML PATH for Grouped String Concatenation

This guide explains how to use SQL Server's STUFF function and FOR XML PATH clause together to efficiently concatenate strings within a group.

Understanding the Components:

1. FOR XML PATH(''): This clause transforms query results into an XML format. Using an empty string ('') as the PATH argument generates a comma-separated list of values from the specified column, enclosed within XML tags. For instance:

   SELECT ',' + name FROM temp1 FOR XML PATH('')

   This would output: ,aaa,bbb,ccc,ddd,eee

2. STUFF Function: The STUFF function modifies a string by replacing a portion of it. We use it here to remove the leading comma produced by FOR XML PATH(''). The function's parameters are:

   • The original string.
   • The starting position for the replacement.
   • The number of characters to remove.
   • The replacement string.

   Thus, STUFF((SELECT ',' NAME FROM temp1 FOR XML PATH('')), 1, 1, '') removes the initial comma, yielding: aaa,bbb,ccc,ddd,eee

3. Joining and Grouping: The final query joins the concatenated string back to the original table using the ID column, grouping the results to achieve the desired output:

   SELECT ID, abc = STUFF(
                (SELECT ',' + name
                 FROM temp1 t1
                 WHERE t1.id = t2.id
                 FOR XML PATH('')), 1, 1, '')
   FROM temp1 t2
   GROUP BY id;

This generates a result set like this:

Id	abc
1	aaa,bbb,ccc,ddd,eee

This method provides a concise and effective way to concatenate strings within groups in SQL Server, avoiding the need for more complex techniques.

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