referencing to "computed columns" in where clause

Is there a way to do things like

select (a-b) as pippo from ... where pippo=7;
or
select
  case
    when (a>3) then a*b
    when (a<3) then a+b
  end as pippo
where pippo<12;

other than defining a function such that I can write:

select func(a,b,c) as pippo from ... where func(a,b,c)=7;
?

Is there anything else I can do to avoid duplication of code?


--
Ivan Sergio Borgonovo
http://www.webthatworks.it