or it shud be like below

create procedure aa

as

begin

if exists (select * from sys.objects where name='bb')

begin

drop procedure bb

declare @a nvarchar(max)

set @a='create procedure bb as begin print ''bb'' end'

exec sp_executesql @a

end

end

But as Gail's suggestion.... why u want to do this??? is this some class room based question???