Question

I wish to modify data by selecting them in a inner query
and count one of them modified..
It gives error..

select count(cvs) from
(
  select 
  cvs,
  (case Citycode when 123 then 'test' else 'other' end) as CityName ,
  (case ProductCode when '000' then 'test3' when 'ss' then 'xtr' else 'ddd' end) as CardName
  from Applications
)

Answer 1

you need to give an alias to the subquery:

select count(x.cvs) from
(
  select 
  cvs,
  (case Citycode when 123 then 'test' else 'other' end) as CityName ,
  (case ProductCode when '000' then 'test3' when 'ss' then 'xtr' else 'ddd' end) as CardName
  from Applications
) x