Can interval take a value from a field?

[Jeff Ross <jross@xxxxxxxxxx>]

Sometimes I think the hurdle to learning more advanced sql is figuring 
out how to phrase the search question.  Well, I'm not having any luck 
there so here I come again to the list, hat in hand :-)

I have a table with an id key and an expiration value in years

  trgexpd_trg_id | expiration_value
----------------+------------------
            240 | 1 year
            749 | 3 years
           4917 | 2 years

and so on.

I'm trying to use the expiration value as the value for an interval in a 
query that looks like:

select
        ts_date as "Transcript Date",
        ts_expiration_date as "Current Expiration Date",
        expiration_value as "Expiration Interval"
        from transcript, training_expiration_value where
        ts_training_id = trgexpd_trg_id and
        ts_training_id in (select cda_training_number from cdas) and
        ts_expiration_date != ts_date + interval 'expiration_value';

and I'm getting the following error:

ERROR:  invalid input syntax for type interval: "expiration_value"

Is there a way to use the value in expiration_value for the interval?

Thanks,

Jeff Ross