思路

题目为“查找购买个数超过20,质量小于50的商品”，其中商品购买个数在交易表trains中进行计算，商品质量在商品表goods中，第一步先求商品购买个数得到查询结果表a，再用商品表左连接表a进行筛选商品。

代码

1.计算商品个数用sum函数

select goods_id,sum(count) total
from trans 
group by goods_id

2.连接2个表格后进行筛选

select g.id,name,weight,total
from goods g left join (select goods_id,sum(count) total from trans group by goods_id) a 
on g.id=a.goods_id
where weight<50 and total>20