Prevent duplicate values in LEFT JOIN

Question

I faced a situation where I got duplicate values from LEFT JOIN. I think this might be a desired behavior but unlike from what I want.

I have three tabl

Answer 1

I think you just need to get lists of departments and phones for particular person. So just use array_agg (or string_agg or json_agg):

SELECT
    p.id,
    p.person_name,
    array_agg(d.department_name) as "department_names",
    array_agg(c.phone_number) as "phone_numbers"
FROM person AS p
LEFT JOIN department AS d ON p.id = d.person_id
LEFT JOIN contact AS c on p.id = c.person_id
GROUP BY p.id, p.person_name