SELECT job, round(SUM(score)/COUNT(job),3) as avg
FROM grade
GROUP BY job
order by avg desc
用group函数按JOB分组，用sum函数计算总分，用count函数计算job数量，用总分除以总数量得平均分avg，round函数对平均分的小数点位数进行处理；最后用order by函数对以上查询结果进行降序排序