Techniques for Returning Multiple Values in a Single Column in T-SQL

Introduction

In SQL Server, a common requirement is to concatenate multiple values from rows into a single column. For instance, in a table UserAliases with columns UserId and Alias, there might be multiple aliases per user, and the goal is to return all aliases for each user in one column, separated by commas.

Main Solution: Using COALESCE with a User-Defined Function

The primary method involves creating a scalar user-defined function that uses the COALESCE function to concatenate strings. This approach is efficient and straightforward for SQL Server 2005.

First, define a function that takes a UserId as input and returns a concatenated string of aliases. The function uses a variable to accumulate the aliases.

CREATE FUNCTION [dbo].[GetAliasesById]
(
    @userID int
)
RETURNS varchar(max)
AS
BEGIN
    declare @output varchar(max)
    select @output = COALESCE(@output + ', ', '') + alias
    from UserAliases
    where userid = @userID

    return @output
END

In this code, the COALESCE function handles the initialization of the @output variable. On the first row, @output is NULL, so COALESCE returns an empty string, effectively starting the concatenation. Subsequent rows add a comma and space before the alias.

To use the function, execute a query that groups by UserId and calls the function for each group.

SELECT UserID, dbo.GetAliasesByID(UserID)
FROM UserAliases
GROUP BY UserID

This will return the desired format with UserId and all aliases concatenated in one column.

Alternative Solution: FOR XML PATH

Another approach is to use the FOR XML PATH clause, which can concatenate strings without creating a user-defined function. This method is useful for ad-hoc queries but may be less efficient for repeated use.

Here is an example query using FOR XML PATH:

;WITH tmp AS ( SELECT DISTINCT UserId FROM UserAliases )
SELECT 
    LEFT(tmp.UserId, 10) +
    '/ ' +
    STUFF(
            (   SELECT ', '+Alias 
                FROM UserAliases 
                WHERE UserId = tmp.UserId 
                FOR XML PATH('') 
            ) 
            , 1, 2, ''
        ) AS [UserId/Alias]
FROM tmp

In this query, the STUFF function is used to remove the leading comma and space added by the FOR XML PATH concatenation. The FOR XML PATH('') concatenates the aliases with the specified separator.

Conclusion

For SQL Server 2005, the user-defined function with COALESCE is recommended as it is reusable and efficient for frequent queries. The FOR XML PATH method is a viable alternative for one-time operations or when avoiding functions. Both methods achieve the goal of aggregating multiple values into a single column, enhancing data presentation in T-SQL queries.