crm_people (person_id, first_name, last_name) holds contacts. Every contact has a first name; some have no last name (last_name is NULL).
Return each contact's full_name for a mailing label: the first name, a single space, then the last name. When the last name is missing, the label is just the first name, with no space after it.
Columns: person_id, full_name. Sort by person_id.