GREATEST and LEAST in SQL standard

My understanding is that GREATEST() and LEAST() are not part of the SQL standard, but are very common.

I'm wondering, is there a way to clone the functionality of GREATEST keeping within the SQL standard?

SELECT id, GREATEST(1,2,3,4,5,6,7) AS number FROM table

The fully query:

  SELECT SUBSTR(section,1,2) AS campus, 
           AVG(GREATEST(maximum - enrolled, 0)) AS empty 
    FROM sectionrun 
   WHERE coursenumber = '105' AND subject = 'ENGL' 
GROUP BY campus