---1.平均銷售等待時間
---有一張Sales表，其中有銷售日期與顧客兩列，現在要求使用一條SQL語句實現計算
--每個顧客的兩次購買之間的平均天數
--假設：在同一個人在一天中不會購買兩次
create table sales
(
custname varchar(10) not null,
saledate datetime not null
)
go
insert sales
select '張三','2010-1-1' union
select '張三','2010-11-1' union
select '張三','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 '趙六','2010-1-1' union
select '錢途','2010-1-1' union
select '錢途','2011-3-1' union
select '張三','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