How to create json tree from postgresql table - json

Question

I have the table with hierarchical data. I want to aggregate the data from one table to a complex json. The elements should be grouped by section column. And nested json should be grouped by subsection column
----------------------------------------------
| id | section | subsection | subsubsection |
----------------------------------------------
| 111 | s_1 | null | null | // root section
----------------------------------------------
| 222 | s_1 | ss_2 | null | // root subsection
----------------------------------------------
| 333 | s_1 | ss_2 | sss_3 |
----------------------------------------------
| 444 | s_1 | ss_2 | sss_4 |
----------------------------------------------
| 555 | s_2 | null | null | // root section
----------------------------------------------
| 666 | s_2 | ss_3 | null | // root subsection
----------------------------------------------
I want to create json trees and save them as view
{
"id": 111,
"section": "s_1",
"subsections": [
{
"id": 222,
"subsection": "ss_2",
"subsections": [
{
"id": 333,
"subsection": "sss_3"
},
{
"id": 444,
"subsection": "sss_4"
}
]
}
]
}
What I tried to do:
create or replace view my_view(
name
) as
SELECT
(
SELECT
json_build_object(
'section', a.section,
'subsections', a.sections
)
FROM (SELECT b.section,
json_agg(
json_build_object(
'subsection', b.subsection,
'subsubsections', b.subsections
)
) AS sections
FROM (SELECT c.section,
c.subsection,
json_agg(
json_build_object(
'subsubsection', c.subsubsection
)
) AS subsections
FROM table AS c
GROUP BY section, subsection
) AS b
GROUP BY b.section) AS a
) AS name;
But in my solution I can't and an id to each tree node. How to add the each node id to result tree?

Answer 1

I recreated your case with:
create table test (id int, section text, subsection text, subsubsection text);
insert into test values (111,'s_1', null, null);
insert into test values (222,'s_1', 's_2', null);
insert into test values (333,'s_1', 's_2', 's_3');
insert into test values (444,'s_1', 's_2', 's_4');
insert into test values (555,'s_5', null, null);
insert into test values (666,'s_5', 's_6', null);
Note: I changed slightly the names of the sections to be unique.
Result (Ghost is null)
id | section | subsection | subsubsection
-----+---------+------------+---------------
111 | s_1 | 👻 | 👻
222 | s_1 | s_2 | 👻
333 | s_1 | s_2 | s_3
444 | s_1 | s_2 | s_4
555 | s_5 | 👻 | 👻
666 | s_5 | s_6 | 👻
(6 rows)
Then I approached the problem in phases:
Get the list of subsubsections and related sections and subsections
SELECT c.section,
c.subsection,
c.subsubsection,
json_build_object(
'id',c.id,
'subsubsection', c.subsubsection
) AS subsubsections
FROM test AS c
WHERE c.subsubsection is not null
Which gives
section | subsection | subsubsection | subsubsections
---------+------------+---------------+---------------------------------------
s_1 | s_2 | s_3 | {"id" : 333, "subsubsection" : "s_3"}
s_1 | s_2 | s_4 | {"id" : 444, "subsubsection" : "s_4"}
(2 rows)
Then, build subsections by joining the query above with a query specific at subsections level
SELECT c.section,
c.subsection,
json_build_object(
'id',c.id,
'subsection', c.subsection,
'subsubsection', json_agg(subsubsections.subsubsections)
) AS subsections
FROM test AS c
left outer join subsubsections on c.section = subsubsections.section and c.subsection = subsubsections.subsection
WHERE c.subsection is not null and c.subsubsection is null
group by c.section,
c.subsection,
c.id
Note: the subsubsections table is the alias to the previous table
Result
section | subsection | subsections
---------+------------+--------------------------------------------------------------------------------------------------------------------------------------
s_1 | s_2 | {"id" : 222, "subsection" : "s_2", "subsubsection" : [{"id" : 333, "subsubsection" : "s_3"}, {"id" : 444, "subsubsection" : "s_4"}]}
s_5 | s_6 | {"id" : 666, "subsection" : "s_6", "subsubsection" : [null]}
(2 rows)
Last, a query at section level, joining the results of the subsection above
SELECT c.section,
json_build_object(
'id',c.id,
'section', c.section,
'subsection', json_agg(subsections.subsections)
) AS subsections
FROM test AS c
left outer join subsections on c.section = subsections.section
WHERE c.subsection is null
group by c.section,
c.id
The whole query is
WITH subsubsections as (
SELECT c.section,
c.subsection,
c.subsubsection,
json_build_object(
'id',c.id,
'subsubsection', c.subsubsection
) AS subsubsections
FROM test AS c
WHERE c.subsection is not null and c.subsubsection is not null
)
, subsections as (
SELECT c.section,
c.subsection,
json_build_object(
'id',c.id,
'subsection', c.subsection,
'subsubsection', json_agg(subsubsections.subsubsections)
) AS subsections
FROM test AS c
left outer join subsubsections on c.section = subsubsections.section and c.subsection = subsubsections.subsection
WHERE c.subsection is not null and c.subsubsection is null
group by c.section,
c.subsection,
c.id
)
SELECT c.section,
json_build_object(
'id',c.id,
'section', c.section,
'subsection', json_agg(subsections.subsections)
) AS subsections
FROM test AS c
left outer join subsections on c.section = subsections.section
WHERE c.subsection is null
group by c.section,
c.id
;
And the result is
section | subsections
---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
s_1 | {"id" : 111, "section" : "s_1", "subsection" : [{"id" : 222, "subsection" : "s_2", "subsubsection" : [{"id" : 333, "subsubsection" : "s_3"}, {"id" : 444, "subsubsection" : "s_4"}]}]}
s_5 | {"id" : 555, "section" : "s_5", "subsection" : [{"id" : 666, "subsection" : "s_6", "subsubsection" : [null]}]}
(2 rows)
Note, this will work only if section and subsection names are "unique" per subtree.