Multiple column concatenation

How to concatenate multiple columns in to a single row? For example:

id   name    car
1    sam     dodge
1    ram     maserati
1    john    benz
1    NULL    mazda
2    kirk    lexus
2    Jim     rolls
1            GMC

The expected result set is:

ID  name               car
1   sam,ram,john       dodge,maserati,benz,mazda,GMC
2   kirk,jim           lexus,rolls

Using a solution I found on Stack Overflow:

SELECT * FROM (
SELECT t.id,stuff([m].query('/name').value('/', 'varchar(max)'),1,1,'') AS [SomeField_Combined1],
stuff([m].query('/car').value('/', 'varchar(max)'),1,1,'') AS [SomeField_Combined2]
FROM dbo.test t
OUTER apply(SELECT (

SELECT id, ','+name AS name
,','+car AS car
FROM test WHERE test.id=t.id
FOR XML PATH('') ,type)
             AS  M) A)S
GROUP BY id,somefield_combined1,somefield_combined2

Are there any better solutions? The inner select comes from an expensive multi-table join (not the single table 'test' shown above). The query is in an in-line TVF, so I cannot use a temporary table.

Also, if there is a blank column the results will yield extra commas
like

ID  name                car
1   sam,ram,john,,      dodge,maserati,benz,mazda,GMC
2   kirk,jim           lexus,rolls

Is there any way to prevent this?

I ran a few tests using a little over 6 mil rows. With an index on the ID column.

Here is what I came up with.

Your initial query:

SELECT * FROM (
    SELECT t.id,
            stuff([M].query('/name').value('/', 'varchar(max)'),1,1,'') AS [SomeField_Combined1],
            stuff([M].query('/car').value('/', 'varchar(max)'),1,1,'') AS [SomeField_Combined2]
    FROM dbo.test t
    OUTER APPLY(SELECT (
                    SELECT id, ','+name AS name
                    ,','+car AS car
                    FROM test WHERE test.id=t.id
                    FOR XML PATH('') ,type)
                 AS  M) 
            M ) S
GROUP BY id, SomeField_Combined1, SomeField_Combined2

This one ran for ~23 minutes.

I ran this version which is the version I first learned. In some ways it seems like it should take longer but it doesn't.

SELECT test.id,
    STUFF((SELECT ', ' + ThisTable.name
            FROM   test ThisTable
            WHERE  test.id = ThisTable.id
            AND    ThisTable.name <> ''
            FOR XML PATH ('')),1,2,'') AS ConcatenatedSomeField,
    STUFF((SELECT ', ' + car
            FROM   test ThisTable
            WHERE  test.id = ThisTable.id
            AND    ThisTable.car <> ''
            FOR XML PATH ('')),1,2,'') AS ConcatenatedSomeField2
FROM test 
GROUP BY id

This version ran in just over 2 minutes.

StackExchange Database Administrators Q#125771, answer score: 7