Create a nested json with column values as key-value pairs - sql

Question

I am trying to build a JSON from the following tables
table : car_makers
+------+-------------+---------+
| cmid | companyname | country |
+------+-------------+---------+
| 1 | Toyota | Japan |
| 2 | Volkswagen | Germany |
| 3 | Nissan | Japan |
+------+-------------+---------+
Table : cars
+------+---------+-----------+
| cmid | carname | cartype |
+------+---------+-----------+
| 1 | Camry | Sedan |
| 1 | Corolla | Sedan |
| 2 | Golf | Hatchback |
| 2 | Tiguan | SUV |
| 3 | Qashqai | SUV |
+------+---------+-----------+
I am trying to create a nested JSON of this structure :
{
"companyName": "Volkswagen",
"carType": "Germany",
"cars": {
"Tiguan": "SUV",
"Golf": "Hatchback"
}
}
but the best I could do with the this query
select json_build_object('companyName',companyName, 'carType', country, 'cars', JSON_AGG(json_build_object('carName', carName, 'carType', carType) ))
from car_makers cm
join cars c on c.cmid = cm.cmid
group by companyName,country
is this -
{
"companyName": "Volkswagen",
"carType": "Germany",
"cars": [
{
"carName": "Tiguan",
"carType": "SUV"
},
{
"carName": "Golf",
"carType": "Hatchback"
}
]
}
So, how can I correct my current query to replace the nested json array with a json element of key-value pairs from column values ?
here is the fiddle with sample data and the query I have tried

Answer 1

You can use json_object_agg:
select json_build_object('companyName', c.companyName,
'country', c.country, 'cars', json_object_agg(c1.carName, c1.carType))
from car_makers c join cars c1 on c.cmid = c1.cmid
group by c.companyName, c.country
See fiddle.