Joining tables and creating a json out of the joined information - sql

Question

Is there a way to join 2 tables with one query to DB in a way when records from one table are put as an array value in a 'new' column in another table?
(It's clear how to do it with 2 queries to both tables and processing results in code, but is there a way to use only one SELECT with joining the tables "during" the query?)
So, here is a simple example:
Table 1:
id
value
1
v1
2
v2
Table 2:
id
id_t1
value
1
1
v3
2
1
v4
3
2
v5
As a query result of selecting all the values from Table 1 joined with Table 2 there should be the next array of objects (to make the example more general id_t1 from Table 2 is filtered from the joined results):
[
{
id: 1,
value: v1,
newColumnForJoinedValuesFromTable2: [ { id: 1, value: v3 }, { id: 2, value: v4} ]
},
{
id: 2,
value: v2,
newColumnForJoinedValuesFromTable2: [ { id: 3, value: v5 } ]
}
]

Answer 1

You can achieve your json by stacking twice the following functions:
JSON_BUILD_OBJECT, to build your jsons, given <key,value> pairs
JSON_AGG, to aggregate your arrays
WITH tab2_agg AS (
SELECT id_t1,
JSON_AGG(
JSON_BUILD_OBJECT('id' , id_,
'value', value_)
) AS tab2_json
FROM tab2
GROUP BY id_t1
)
SELECT JSON_AGG(
JSON_BUILD_OBJECT('id' , id_,
'value' , value_,
'newColumnForJoinedValuesFromTable2', tab2_json)
) AS your_json
FROM tab1
INNER JOIN tab2_agg
ON tab1.id_ = tab2_agg.id_t1
Check the demo here.

Answer 2

Use json_agg(json_build_object(...)) and group by.
select json_agg(to_json(t)) as json_result from
(
select t1.id, t1.value,
json_agg(json_build_object('id',t2.id,'value',t2.value)) as "JoinedValues"
from t1 join t2 on t2.id_t1 = t1.id
group by t1.id, t1.value
) as t;
See demo