[SQL] Unable to identify an ordering operator

[Ferruccio Zamuner]

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 the query


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