It’s as simple as:
select zip_code, (array_agg(company_name))[1]
from locations
group by zip_code
;
zip_code | array_agg
----------+--------------------------
90210 | In-N-out
46368 | Johnny's Round the Clock
(2 rows)
source:
create table locations (
id bigint generated by default as identity primary key,
zip_code text not null,
company_name text not null
);
insert into locations (zip_code, company_name) values
('46368', 'Johnny''s Round the Clock'),
('90210', 'In-N-out'),
('46368', 'Albanese Candy');