Row concatenation with FOR XML, but with multiple columns?

Question

I often use queries like:

SELECT *
FROM   ThisTable
       OUTER APPLY (SELECT (SELECT SomeField + \' \' AS [data()]
                            FROM   SomeT         


        

Answer 1

This is the same answer as I gave here: https://dba.stackexchange.com/questions/125771/multiple-column-concatenation/

The OP of that question referenced the answer given here. You can see below that sometimes the simplest answer can be the best. If SomeTable is multiple tables then I would go ahead and put it into a CTE to avoid having the same complex code multiple times.

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

This version ran in just over 2 minutes.