postgresql How to generate nested json objects by one table - json

Question

I want to aggregate the data from one table to a complex json. The elements should be grouped by id column. And nested json should be grouped by section column
I have a table with data:
-----------------------------
| id | section | subsection |
----------------------------
| 1 | s_1 | ss_1 |
----------------------------
| 1 | s_1 | ss_2 |
----------------------------
| 1 | s_2 | ss_3 |
----------------------------
| 2 | s_3 | ss_4 |
----------------------------
I want to create json like that:
[
{
"id": 1,
"sections": [
{
"section": "s_1",
"subsections": [
{
"subsection": "ss_1"
},
{
"subsection": "ss_2"
}
]
},
{
"section": "s_2",
"subsections": [
{
"subsection": "ss_3"
}
]
}
]
},
{
"id": 2,
"sections": [
{
"section": "s_3",
"subsections": [
{
"subsection": "ss_3"
}
]
}
]
}
]
I tried do it like that:
select
json_build_array(
json_build_object(
'id', a.id,
'sections', json_agg(
json_build_object(
'section', b.section,
'subsections', json_agg(
json_build_object(
'subsection', c.subsection
)
)
)
)
)
)
from table as a
inner join table as b on a.section = b.section
inner join table as c on b.subsection = c.subsection
group by a.id;
BUT there is a problem: Nested aggregate calls are not allowed
Is there any possible way to use nested aggregate calls? Or is there more elegant solution?

Answer 1

You'll need to use CTEs or nested queries for this. No JOINs necessary though:
SELECT json_agg(
json_build_object(
'id', a.id,
'sections', a.sections
)
)
FROM (
SELECT b.id, json_agg(
json_build_object(
'section', b.section,
'subsections', b.subsections
)
) AS sections
FROM (
SELECT c.id, c.section, json_agg(
json_build_object(
'subsection', c.subsection
)
) AS subsections
FROM table AS c
GROUP BY id, section
) AS b
GROUP BY id
) AS a;