PostgreSQL: Get an entity with all his relationships - sql

Question

I have a table "Cars" and a table "Person". A Person drives many Cars and a Car can be driven by many People so I have another table "Person_Car" which has both id's per row.
Car(id, name)
Person(id, name)
Person_Car(car_id, person_id)
How can I get a list of all people with the cars it drives (car names concatenated), something like this:
("John", "Car 1, Car 2, Car 3")
("Kate", "Car 2, Car 4, Car 5")

Answer 1

Example is here: http://sqlfiddle.com/#!15/ba949/1
Test data:
Create table Car(id int, name text);
Create table Person(id int, name text);
Create table Person_Car(car_id int, person_id int);
INSERT INTO Car VALUES (1, 'Car 1'),
(2, 'Car 2'),
(3, 'Car 3'),
(4, 'Car 4'),
(5, 'Car 5');
INSERT INTO Person VALUES(1, 'John'), (2, 'Kate');
INSERT INTO Person_Car VALUES (1,1), (2,1), (3,1), (2,2), (4,2), (5,2);
Your desired code:
SELECT p.name, array_to_string(array_agg(c.name), ',') FROM Person p
INNER JOIN Person_Car pc ON p.id=pc.person_id
INNER JOIN Car c ON c.id=pc.car_id
GROUP by p.name
Output:
John Car 1,Car 2,Car 3
Kate Car 2,Car 4,Car 5

Answer 2

Just in case you want to avoid the GROUP BY
Option 1
WITH forienKeyTable AS
(
SELECT pc.person_id, c.name
FROM Car c
JOIN Person_Car pc ON pc.car_id = c.id
)
SELECT p.name
, array_to_string
(ARRAY(
SELECT fkt.name
FROM forienKeyTable fkt
WHERE fkt.person_id = p.id
)::text[], ','::text, 'empty'::text)
FROM Person p;
Option 2
SELECT p.name
, array_to_string
(ARRAY(
SELECT c.name
FROM Car c
JOIN Person_Car pc ON pc.car_id = c.id
WHERE pc.person_id = p.id
)::text[], ','::text, 'empty'::text)
FROM Person p;