Unable to identify an ordering operator

Hello,

select persons.name,firm.name,persons.tel      from persons,work,firm       where (persons.table_owner=0) and
 (work.id_firm=firm.id and work.id_person=persons.id)          union  select persons.name,firm.name,persons.tel
from persons,work,firm          where (persons.table_owner=1) and                (work.id_firm=firm.id and
work.id_person=persons.id);

ERROR:  Unable to identify an ordering operator '<' for type '_text'       Use an explicit ordering operator or modify
thequery
 


I know that this query can be rewrite to eliminate the UNION using an
OR operator on first condition but my question is:

How can I specify an ordering operator? On which field have I to put it?


Best wishes for the new YEAR,              \fer