How to Group Data and Concatenate Values with Comma Separators in SQL?

SQL Grouping and Concatenation with Comma Separators

This guide demonstrates how to write a SQL query that groups data and concatenates aggregated values using comma separators. Imagine a table like this:

<code>| ID | Value |
|-----|-------|
| 1   | a     |
| 1   | b     |
| 2   | c     |</code>

The goal is to generate a result set where values for the same ID are combined into a single comma-separated string:

<code>| ID | Value |
|-----|-------|
| 1   | a,b   |
| 2   | c     |</code>

Solution using FOR XML PATH:

The FOR XML PATH construct provides an efficient way to accomplish this:

SELECT
    ID,
    STUFF((SELECT ', ' + Value
           FROM YourTable t2 WHERE t1.ID = t2.ID
           FOR XML PATH(''), TYPE).value('.', 'NVARCHAR(MAX)'), 1, 2, '') AS Values
FROM YourTable t1
GROUP BY ID;

The STUFF function removes the leading comma and space added by the SELECT statement. This approach is generally preferred for its performance and readability.

Further Reading:

For more advanced scenarios and alternative techniques, explore these resources:

• Stack Overflow: A relevant Stack Overflow thread discussing similar concatenation challenges (search for "SQL group by concatenate").
• SQL Server Documentation: Consult the official Microsoft documentation for detailed information on FOR XML PATH and related functions.

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