Extract JSON values recursivey in postgres - sql

Question

I have some JSON data stored in a column. I want to parse the json data and extract all the values against a particular key.
Here's my sample data:
{
"fragments": [
{
"fragments": [
{
"fragments": [
{
"fragments": [],
"fragmentName": "D"
},
{
"fragments": [],
"fragmentName": "E"
},
{
"fragments": [],
"fragmentName": "F"
}
],
"fragmentName": "C"
}
],
"fragmentName": "B"
}
],
"fragmentName": "A"
}
Expected output:
D, E, F, C, B, A
I want to extract all fragmentName values from the above JSON.
I have gone through the below stacks, but haven't found anything useful:
Collect Recursive JSON Keys In Postgres
Postgres recursive query with row_to_json
Edited:
Here's one approach I have tried on the above stacks:
WITH RECURSIVE key_and_value_recursive(key, value) AS (
SELECT
t.key,
t.value
FROM temp_frg_mapping, json_each(temp_frg_mapping.info::json) AS t
WHERE id=2
UNION ALL
SELECT
t.key,
t.value
FROM key_and_value_recursive,
json_each(CASE
WHEN json_typeof(key_and_value_recursive.value) <> 'object' THEN '{}' :: JSON
ELSE key_and_value_recursive.value
END) AS t
)
SELECT *
FROM key_and_value_recursive;
Output:
Getting only 0 level nesting.

Answer 1

I would use a recursive query, but with jsonb_array_elements():
with recursive cte as (
select id, info ->> 'fragmentName' as val, info -> 'fragments' as info, 1 lvl
from mytable
where id = 2
union all
select c.id, x.info ->> 'fragmentName', x.info -> 'fragments', c.lvl + 1
from cte c
cross join lateral jsonb_array_elements(c.info) as x(info)
where c.info is not null
)
select id, val, lvl
from cte
where val is not null
The query traverses the object depth-first; at each step of the way, we unnest the json array and check if a fragment name is available. We don't need to check the types of the returned values: we just use the standard functions, until the data exhausts.
Demo on DB Fiddle
Sample data:
{
"fragments": [
{
"fragments": [
{
"fragments": [
{
"fragments": [
],
"fragmentName": "D"
},
{
"fragments": [
],
"fragmentName": "E"
},
{
"fragments": [
],
"fragmentName": "F"
}
],
"fragmentName": "C"
}
],
"fragmentName": "B"
}
],
"fragmentName": "A"
}
Results:
id | val | lvl
-: | :-- | --:
2 | A | 1
2 | B | 2
2 | C | 3
2 | D | 4
2 | E | 4
2 | F | 4