how to retrieve data from multiple tables (postgresql) - postgresql

Question

I have 4 different tables that are linked to each other in the following way (I only kept the essential columns in each table to emphasise the relationships between them):
create TABLE public.country (
country_code varchar(2) NOT NULL PRIMARY KEY,
country_name text NOT NULL,
);
create table public.address
(
id integer generated always as identity primary key,
country_code text not null,
CONSTRAINT FK_address_2 FOREIGN KEY (country_code) REFERENCES public.country (country_code)
);
create table public.client_order
(
id integer generated always as identity primary key,
address_id integer null,
CONSTRAINT FK_client_order_1 FOREIGN KEY (address_id) REFERENCES public.address (id)
);
create table public.client_order_line
(
id integer generated always as identity primary key,
client_order_id integer not null,
product_id integer not null,
client_order_status_id integer not null default 0,
quantity integer not null,
CONSTRAINT FK_client_order_line_0 FOREIGN KEY (client_order_id) REFERENCES public.client_order (id)
);
I want to get the data in the following way: for each client order line to show the product_id, quantity and country_name(corresponding to that client order line).
I tried this so far:
SELECT country_name FROM public.country WHERE country_code = (
SELECT country_code FROM public.address WHERE id = (
SELECT address_id FROM public.client_order WHERE id= 5
)
)
to get the country name given a client_order_id from client_order_line table. I don't know how to change this to get all the information mentioned above, from client_order_line table which looks like this:
id client_order_id. product_id. status. quantity
1 1 122 0 1000
2 2 122 0 3000
3 2 125 0 3000
4 3 445 0 2000
Thanks a lot!

Answer 1

You need a few join-s.
select col.client_order_id,
col.product_id,
col.client_order_status_id as status,
col.quantity,
c.country_name
from client_order_line col
left join client_order co on col.client_order_id = co.id
left join address a on co.address_id = a.id
left join country c on a.country_code = c.country_code
order by col.client_order_id;
Alternatively you can use your select query as a scalar subquery expression.