select a.商品号,a.尺码,round(sum(b.库存数)/sum(a.近7天销量),4) as 存销比
from 订单表 as a
left join 库存表 as b on a.商品号=b.商品号
and a.尺码=b.尺码
group by a.商品号,a.尺码
order by a.商品号 asc,field(a.尺码,'S','M','L','XL');