1.
select s.gid,sum(s.num*g.price) as mm
from sales s,goods g
where s.gid=g.gid
group by s.gid


2.

 select b.* from buyer b,sales ss where b.bid=ss.bid and b.bid not in(
   select bid from sales s where exists
 (select 1 from goods g where s.gid=g.gid and g.name='A')
)

3.

select s.bid,b.name,sum(s.num*g.price) as mm
from sales s,goods g,buyer b
where s.gid=g.gid and b.bid=s.bid
group by s.bid,b.name
order by 3
limit 3

------解决方案--------------------
贴错了。。。重新贴一次
SQL code

1.
--如果销售记录在sales为空则total = 0
select a.gid,a.name,sum(ifnull(a.price*b.num,0)) as total 
from goods a left join sales b on a.gid = b.gid 
group by a.gid

------解决方案--------------------
探讨

引用:
(不要高估你的汉语表达能力或者我的汉语理解能力)
建议你列出你的表结构，并提供测试数据以及基于这些测试数据的所对应正确结果。
参考一下这个贴子的提问方式http://topic.csdn.net/u/20091130/20/8343ee6a-417c-4c2d-9415-fa46604a00cf.html

1. 你的 create tabl……


------解决方案--------------------
1：
  
SQL code

SELECT a.gid,(a.price*b.total) AS total FROM goods a LEFT JOIN (SELECT gid,SUM(num) AS total FROM sales GROUP BY gid) b ON a.gid=b.gid