How can I pull a list of ID’s from a SQL table as a comma-separated values string?

MySQL

SELECT GROUP_CONCAT(t.prodid SEPARATOR ',')
  FROM PRODUCTS t
 WHERE t.prodtype="XYZ"

Oracle:

There is an excellent summary of the available string aggregation techniques on Tim Hall’s site.

SQL Server 2005+

SELECT STUFF((SELECT ','+ t.prodid
                FROM PRODUCTS t
               WHERE t.prodtype="XYZ"
             FOR XML PATH('')), 1, 1, '')