Fwd: concatenate and use as field

[Alain Roger <raf.news@xxxxxxxxx>]

Hi,

i have a character varying variable and i concatenate with some other variable, using the '||' operator.
the result of this concatenation should be the name of a column in my table.

however i don't know how to tell that this new concatenated string is a column name.
how to do it ?
thanks.

in fact i'm trying to do the following thing:

select id, theme_ || $1 from themes;

and based on the parameter my stored procedure has received, it should create something like that:
select id, theme_eng from themes;

if $1 = 'eng'