---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