MySQL: get MAX or GREATEST of several columns, but with NULL fields

Use COALESCE

SELECT id, 
   GREATEST(date1, 
     COALESCE(date2, 0),
     COALESCE(date3, 0)) as datemax 
FROM mytable

Update: This answer previously used IFNULL which does work, but as Mike Chamberlain pointed out in the comments, COALESCE is actually the preferred method.