If condition in Where Clause in SQL Query(Please help)

hi,

try

create table #temp
(
ID int,
Username varchar(20),
Status varchar(10),
Date1 date,
Date2 date,
date3 date
)
insert into #temp values(101,'abc','Clear','2012-11-01','2013-01-01','2013-01-01'),
 (102,'def','Rejected','2012-12-03','2013-01-05','2013-01-01'),
 (103,'ghi','Clear','2011-02-04','2012-03-04','2013-01-01'), 
 (104,'jkl','Rejected','2012-02-03','2013-01-01','2013-01-01'), 
 (105,'mno','Rejected','2013-01-01','2013-01-01','2013-01-01'), 
 (106,'pqr','Clear','2013-01-01','2013-04-01','2013-01-01') 
 
 drop table #temp
 
 select Date1,Date2,Date3,Status,Username,
 case Status when 'Clear' then datediff(DAY,Date2,Date1) 
 else datediff(DAY,Date3,Date2) end as TotalDays
 from #temp

hope this will help you.