[sql] How Stuff and 'For Xml Path' work in SQL Server?

This article covers various ways of concatenating strings in SQL, including an improved version of your code which doesn't XML-encode the concatenated values.

SELECT ID, abc = STUFF
(
    (
        SELECT ',' + name
        FROM temp1 As T2
        -- You only want to combine rows for a single ID here:
        WHERE T2.ID = T1.ID
        ORDER BY name
        FOR XML PATH (''), TYPE
    ).value('.', 'varchar(max)')
, 1, 1, '')
FROM temp1 As T1
GROUP BY id

To understand what's happening, start with the inner query:

SELECT ',' + name
FROM temp1 As T2
WHERE T2.ID = 42 -- Pick a random ID from the table
ORDER BY name
FOR XML PATH (''), TYPE

Because you're specifying FOR XML, you'll get a single row containing an XML fragment representing all of the rows.

Because you haven't specified a column alias for the first column, each row would be wrapped in an XML element with the name specified in brackets after the FOR XML PATH. For example, if you had FOR XML PATH ('X'), you'd get an XML document that looked like:

<X>,aaa</X>
<X>,bbb</X>
...

But, since you haven't specified an element name, you just get a list of values:

,aaa,bbb,...

The .value('.', 'varchar(max)') simply retrieves the value from the resulting XML fragment, without XML-encoding any "special" characters. You now have a string that looks like:

',aaa,bbb,...'

The STUFF function then removes the leading comma, giving you a final result that looks like:

'aaa,bbb,...'

It looks quite confusing at first glance, but it does tend to perform quite well compared to some of the other options.