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