select case when Platform='C' then 'CS-'+Description
            when Platform='W' then 'Web-'+Description end
from tb

------解决方案--------------------
SQL code

declare @t table (ID int,Description varchar(3),Platform varchar(1))
insert into @t
select 1,'aaa','C' union all
select 2,'bbb','W' union all
select 3,'ccc','C' union all
select 4,'ddd','W' union all
select 5,'eee','W'

select ID,C1+Description as Description from @t a 
left join (select 'CS-' AS C1 UNION SELECT 'WEB-') b on a.Platform=left(b.C1,1)
/*
ID          Description
----------- -----------
1           CS-aaa
2           WEB-bbb
3           CS-ccc
4           WEB-ddd
5           WEB-eee
*/