PostgreSQL - How to convert seconds in a numeric field to HH:MM:SS

SELECT  TO_CHAR('1000 second'::interval, 'HH24:MI:SS')

or, in your case

SELECT  TO_CHAR((mycolumn || ' second')::interval, 'HH24:MI:SS')
FROM    mytable

To convert seconds to HH:MM:SS

SELECT 120 * interval '1 sec';

You will get 00:02:00

To convert minutes to HH:MM:SS

SELECT 120 * interval '1 min'; 

You will get 02:00:00