查询各客户订购的某同一产品的总数和平均单价

求sqlserver语句的实现
插入ORDERS表Dates，Cno客户编号，Pno产品编号，Amount数量

select Cno,Pno,count(Amount) as 总数,avg(Price) as 平均单价
from Dates group by Cno,Pno where Pno=‘’