oracle sql问题

现在要做一个查询,查询所有用户中weight的和中最大的用户的custname 本人新手 跪求高人指点，可能要做聚集函数嵌套之类的 只是语法是在不是很熟悉


custom表  custid custname   shipstuff表   custid   weight  中
                 a001  d1                                  a001    500
                 a002  d2                                  a003    300
                 a003  d3                                  a001    400
                                                                 a002    400
                                                                 a003    200

追问：能给出每条语句的详细解释不  谢谢了能给出每条语句的详细解释不  谢谢了

答案：select * 
  from custom
 where custid = (
	select custid 
	  from (select custid, sum(weight) as val 
		  from shipstuff group by custid)
	 where rowunm=1
	 order by val desc
 )

其他：select custname  from custom
 where custid = (select custid 
	  from (select custid, sum(weight) wt from shipstuff 
                                  group by custid 
                                  order by val desc)
	 where rownum=1
	  ) 这样的表结构  没法看啊 

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