Get data from different tables indexed by same ID

Question

Database: postgresql 12. I have a table people:

person_id | name
----------+-----
1         |John
2         |Paul
3         |Jim
...

Each person can have zero or more car insurance contracts, so I have another table car_contracts as follows:

contract_id | person_id | contract_number | date_from | insured_value | car_tag | .... (more fields)
------------+-----------+-----------------+-----------+---------------+---------+ 
1           | 1         | C99             |2020-01-01 | 5000          | ABC     | ....
2           | 1         | C02             |2020-03-01 | 4000          | XYZ     | ....
3           | 3         | C96             |2020-02-02 | 12000         | KKK     | ....
4           | 1         | C01             |2019-01-01 | 5500          | XXX     | ....
5           | 2         | C02             |2019-06-01 | 1000          | 666     | ....
6           | 3         | C03             |2020-02-20 | 11000         | KPX     | ....
...

Each person can also have zero or more power plants (or whatever; the point is, another table with different fields but still tied to people by means of person_id), so the table plants is like

plant_id | person_id | plant_name | power | address     | .... (more fields)
---------+-----------+------------+-------+-------------+ 
1        | 2         | plant01    |100    | 230 rue st. | ....
2        | 3         | jimsplant1 |50     | ...         | ....
3        | 3         | jimsplant2 |65.5   |             | ....
4        | 2         | plant02    |12.5   |             | ....
5        | 1         | house01    |20     |             | ....
6        | 1         | house02    |20     |             | ....
...

I can either get a list of people with their contracts or people with their plants by joining the people table to the car_contracts and plants table respectively. Is it possible to somehow get all the information at once (people with their contracts and their plants) with a single query?

The main issue seems to me that the number of contracts and the number of plants have no relation and can be very different, so I think joining people with contracts with plants makes little sense.

Answer 1

Well, you can aggregate the rows into arrays:

select p.person_id, p.name, 
       array_agg(cc) as contracts,
       array_agg(pl) as plants
from people p
  left join car_contracts cc on p.person_id = cc.person_id
  left join plants pl on p.person_id = pl.person_id
group by p.person_id;

(This assumes that person_id is declared as the primary key)

Aggregating the rows into json values might be easier to work with in the front end:

select p.person_id, p.name, 
       json_agg(to_jsonb(cc)) as contracts,
       jsong_agg(to_jsonb(pl)) as plants
from people p
  left join car_contracts cc on p.person_id = cc.person_id
  left join plants pl on p.person_id = pl.person_id
group by p.person_id;

To avoid duplicates, do the aggregation in a derived table:

select p.person_id, p.name, 
       cc.contracts,
       pl.plants
from people p
  left join (
    select person_id, jsonb_agg(to_jsonb(c)) as contracts
    from car_contracts c
    group by person_id
  ) cc on p.person_id = cc.person_id
  left join (
    select person_id, jsonb_agg(to_jsonb(p)
    from plants 
    group by person_id
  ) pl on p.person_id = pl.person_id;