casting... adding integer to timestamp - Mailing list pgsql-general

If play_length is a timestamp, I can do this:

SELECT  play_length - INTERVAL '13 seconds' ...

But what if play_seconds is a column?

SELECT  play_length - INTERVAL 'play_seconds seconds' ...

This doesn't work.

ERROR:  invalid input syntax for type interval: "play_seconds seconds"

Can anyone help?

Thanks,
Mark