select
    a.商品号,
    a.尺码,
    sum(b.库存数) / sum(a.近7天销量) as 存销比
from
    订单表 a 
left join
    库存表 b on a.商品号 = b.商品号 and a.尺码 = b.尺码
group by 
    1,2
order by 
    1,field(a.尺码,'S','M','L','XL')