How to create nested JSON return with aggregate function and dynamic key values using `jsonb_build_object` - sql

Question

This is what and example of the table looks like.
+---------------------+------------------+------------------+
| country_code | region | num_launches |
+---------------------+------------------+------------------+
| 'CA' | 'Ontario' | 5 |
+---------------------+------------------+------------------+
| 'CA' | 'Quebec' | 9 |
+---------------------+------------------+------------------+
| 'DE' | 'Bavaria' | 15 |
+---------------------+------------------+------------------+
| 'DE' | 'Saarland' | 12 |
+---------------------+------------------+------------------+
| 'DE' | 'Berlin' | 23 |
+---------------------+------------------+------------------+
| 'JP' | 'Tokyo' | 19 |
+---------------------+------------------+------------------+
I am able to write a query that returns each country_code with all regions nested within, but I am unable to get exactly what I am looking for.
My intended return looks like.
[
{ 'CA': [
{ 'Ontario': 5 },
{ 'Quebec': 9 }
]
},
{ 'DE': [
{ 'Bavaria': 15 },
{ 'Saarland': 12 },
{ 'Berlin': 23 }
]
},
{ 'JP': [
{ 'Tokyo': 19 }
]
}
]
How could this be calculated if the num_launches was not available?
+---------------------+------------------+
| country_code | region |
+---------------------+------------------+
| 'CA' | 'Ontario' |
+---------------------+------------------+
| 'CA' | 'Ontario' |
+---------------------+------------------+
| 'CA' | 'Ontario' |
+---------------------+------------------+
| 'CA' | 'Quebec' |
+---------------------+------------------+
| 'CA' | 'Quebec' |
+---------------------+------------------+
| 'DE' | 'Bavaria' |
+---------------------+------------------+
| 'DE' | 'Bavaria' |
+---------------------+------------------+
| 'DE' | 'Bavaria' |
+---------------------+------------------+
| 'DE' | 'Bavaria' |
+---------------------+------------------+
| 'DE' | 'Saarland' |
+---------------------+------------------+
| 'DE' | 'Berlin' |
+---------------------+------------------+
| 'DE' | 'Berlin' |
+---------------------+------------------+
| 'JP' | 'Tokyo' |
+---------------------+------------------+
Expected Return
[
{ 'CA': [
{ 'Ontario': 3 },
{ 'Quebec': 2 }
]
},
{ 'DE': [
{ 'Bavaria': 4 },
{ 'Saarland': 1 },
{ 'Berlin': 2 }
]
},
{ 'JP': [
{ 'Tokyo': 1 }
]
}
]
Thanks

Answer 1

You can try to use json_agg with json_build_object function in a subquery to get the array then do it again in the main query.
Schema (PostgreSQL v9.6)
CREATE TABLE T(
country_code varchar(50),
region varchar(50),
num_launches int
);
insert into t values ('CA','Ontario',5);
insert into t values ('CA','Quebec',9);
insert into t values ('DE','Bavaria',15);
insert into t values ('DE','Saarland',12);
insert into t values ('DE','Berlin',23);
insert into t values ('JP','Tokyo',19);
Query #1
select json_agg(json_build_object(country_code,arr)) results
from (
SELECT country_code,
json_agg(json_build_object(region,num_launches)) arr
FROM T
group by country_code
) t1;
results
[{"CA":[{"Ontario":5},{"Quebec":9}]},{"DE":[{"Bavaria":15},{"Saarland":12},{"Berlin":23}]},{"JP":[{"Tokyo":19}]}]
View on DB Fiddle