明确问题：购买个数超过20，质量小于50的商品，按照商品id升序排序
首先要求和每个goods_id购买了多少个
  •  select goods_id,sum(count) total
  • from trans 
  •  group by goods_id
在与goods进行连接，设定条件即可
  • select b.id,b.name,b.weight,a.total
  • from (select goods_id,sum(count) total
  • from trans 
  •  group by goods_id) a
  •  join  goods as b
  •  on a.goods_id=b.id
  •  where b.weight<50 and  a.total>20