SQL语句练习实例之三平均销售等待时间

有一张Sales表，其中有销售日期与顾客两列，现在要求使用一条SQL语句实现计算

---1.平均销售等待时间 
---有一张Sales表，其中有销售日期与顾客两列，现在要求使用一条SQL语句实现计算 
--每个顾客的两次购买之间的平均天数 
--假设：在同一个人在一天中不会购买两次 
create table sales 
( 
custname varchar(10) not null, 
saledate datetime not null 
) 
go 
insert sales 
select '张三','2010-1-1' union 
select '张三','2010-11-1' union 
select '张三','2011-1-1' union 
select '王五','2010-2-1' union 
select '王五','2010-4-1' union 
select '李四','2010-1-1' union 
select '李四','2010-5-1' union 
select '李四','2010-9-1' union 
select '李四','2011-1-1' union 
select '赵六','2010-1-1' union 
select '钱途','2010-1-1' union 
select '钱途','2011-3-1' union 
select '张三','2011-9-1' 
go 
select custname,DATEDIFF(d,min(saledate),max(saledate))/(COUNT(*)-1) as avgday 
from sales 
group by custname 
having count(*)>1 
go 
select custname,case when count(*)>1 then DATEDIFF(d,min(saledate),max(saledate))/(COUNT(*)-1) 
else DATEDIFF(d,min(saledate),max(saledate)) end 
as avgday 
from sales 
group by custname 
--having count(*)>1 
go 
drop table sales