SQL语句练习实例之三——平均销售等待时间

---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