SQL标准中的最大和最小值

您可以使用 CASE 表达式:

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

GREATEST(1,2,3,4,5,6,7) AS number

可以变成

(select max(tmp) from (
        select 1 tmp from dual
        union all
        select 2 tmp from dual
        union all
        select 3 tmp from dual
        union all
        select 4 tmp from dual
        union all
        select 5 tmp from dual
        union all
        select 6 tmp from dual
        union all
        select 7 tmp from dual
) ) AS number