use case in where clause in sqlserver 2005

steps:

–declare temp table
declare@tbl as table(id int,username varchar(50),Age int)
–insert data into temp table
insert into @tbl
values
(1,‘Amit’,10) 

insert into @tbl
 

insert into @tbl

values
(1,‘Bhavin’,10) 

insert into @tbl
 

insert into @tbl

values
(1,‘Chirag’,10) 

insert into @tbl
 

insert into @tbl

values
(1,‘Dhaval’,10) 

insert into @tbl
 

insert into @tbl

values
(1,‘Nirav’,10) 

insert into @tbl
 

insert into @tbl

values
(1,‘kumar’,10)

–declare variable
declare @username varchar(50)
set @username =‘a’ 

select * from @tbl  

select * from @tbl

where
case @username
     whennullthen1
     when‘a’then
          case whenusername like‘a%’then1 else0 end
     when‘b’then
            case whenusername like‘b%’then1 else0 end
end =1