using calculated column in where-clause

Hi!

I'd like to do some calculation with values from the table, show them a
new column and use the values in a where-clause.

Something like this
select a, b , a*b as c from ta where c=2;

But postgresql complains, that column "c" does not exist.

Do I have to repeat the calculation (which might be even more complex
:-) ) in the "where"-clause, or is there a better way?


Thanks in advance.

Best regards
Patrick