SQL 문장 연습 실례의 세 번째―평균 판매 대기 시간

 
  
---1.         
  
---   Sales ，            ，        SQL       
  
--                 
  
--  ：                
  
create table sales 
  
( 
  
custname varchar(10) not null, 
  
saledate datetime not null 
  
) 
  
go 
  
insert sales 
  
select '  ','2010-1-1' union 
  
select '  ','2010-11-1' union 
  
select '  ','2011-1-1' union 
  
select '  ','2010-2-1' union 
  
select '  ','2010-4-1' union 
  
select '  ','2010-1-1' union 
  
select '  ','2010-5-1' union 
  
select '  ','2010-9-1' union 
  
select '  ','2011-1-1' union 
  
select '  ','2010-1-1' union 
  
select '  ','2010-1-1' union 
  
select '  ','2011-3-1' union 
  
select '  ','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