Combine column from multiple rows into single row

I've got some customer_comments split out into multiple rows due to database design, and for a report I need to combine the comments from each unique id into one row. I previously tried something working with this delimited list from SELECT clause and COALESCE trick but I can't recall it and must not have saved it. I can't seem to get it to work in this case either, only seems to work on a single row.

The data looks like this:

id  row_num  customer_code comments
-----------------------------------
1   1        Dilbert        Hard
1   2        Dilbert        Worker
2   1        Wally          Lazy

My results need to look like this:

id  customer_code comments
------------------------------
1   Dilbert        Hard Worker
2   Wally          Lazy

So for each row_num there's really only one row of results; the comments should be combined in the order of row_num. The above linked SELECT trick works to get all the values for a specific query as one row, but I can't figure out how to make it work as part of a SELECT statement that spits all these rows out.

My query has to go through the whole table on its own and output these rows. I'm not combining them into multiple columns, one for each row, so PIVOT doesn't seem applicable.

This is relatively trivial to do with a correlated subquery. You can't use the COALESCE method highlighted in the blog post you mention unless you extract that to a user-defined function (or unless you only want to return one row at a time). Here is how I typically do this:

DECLARE @x TABLE 
(
  id INT, 
  row_num INT, 
  customer_code VARCHAR(32), 
  comments VARCHAR(32)
);

INSERT @x SELECT 1,1,'Dilbert','Hard'
UNION ALL SELECT 1,2,'Dilbert','Worker'
UNION ALL SELECT 2,1,'Wally','Lazy';

SELECT id, customer_code, comments = STUFF((SELECT ' ' + comments 
    FROM @x AS x2 WHERE id = x.id
     ORDER BY row_num
     FOR XML PATH('')), 1, 1, '')
FROM @x AS x
GROUP BY id, customer_code
ORDER BY id;

If you have a case where the data in comments could contain unsafe-for-XML characters (>, <, &), you should change this:

FOR XML PATH('')), 1, 1, '')

To this more elaborate approach:

FOR XML PATH(''), TYPE).value(N'(./text())[1]', N'varchar(max)'), 1, 1, '')

(Be sure to use the right destination data type, varchar or nvarchar, and the right length, and prefix all string literals with N if using nvarchar.)

StackExchange Database Administrators Q#17921, answer score: 18