Extract JSON values recursivey in postgres

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