Flatten nested JSON structure in PostgreSQL - json

Question

I'm trying to write a Postgres query that will output my json data in a particular format.
JSON data structure
{
user_id: 123,
data: {
skills: {
"skill_1": {
"title": "skill_1",
"rating": 4,
"description": 'description text'
},
"skill_2": {
"title": "skill_2",
"rating": 2,
"description": 'description text'
},
"skill_3": {
"title": "skill_3",
"rating": 5,
"description": 'description text'
},
...
}
}
}
This is how I need the data to be formatted in the end:
[
{
user_id: 123,
skill_1: 4,
skill_2: 2,
skill_3: 5,
...
},
{
user_id: 456,
skill_1: 1,
skill_2: 3,
skill_3: 4,
...
}
]
So far I'm working with a query that looks like this:
SELECT
user_id,
data#>>'{skills, "skill_1", rating}' AS "skill_1",
data#>>'{skills, "skill_2", rating}' AS "skill_2",
data#>>'{skills, "skill_3", rating}' AS "skill_3"
FROM some_table
There has to be a better way to go about writing my query. There are 400+ rows and 70+ skills. My above query is a little crazy. Any guidance or help would be greatly appreciated.
Some things to note:
Users rated themselves on 70+ skills
Each skill object has the same structure
Each user rated themselves on the exact same set of skills

Answer 1

db<>fiddle
I expanded your test data to (note the array around all users):
[{
"user_id": 123,
"data": {
"skills": {
"skill_1": {
"title": "skill_1",
"rating": 4,
"description": "description text"
},
"skill_2": {
"title": "skill_2",
"rating": 2,
"description": "description text"
},
"skill_3": {
"title": "skill_3",
"rating": 5,
"description": "description text"
}
}
}
},
{
"user_id": 456,
"data": {
"skills": {
"skill_1": {
"title": "skill_1",
"rating": 1,
"description": "description text"
},
"skill_2": {
"title": "skill_2",
"rating": 3,
"description": "description text"
},
"skill_3": {
"title": "skill_3",
"rating": 4,
"description": "description text"
}
}
}
}]
The query:
SELECT
jsonb_pretty(jsonb_agg(user_id || skills)) -- E
FROM (
SELECT
json_build_object('user_id', user_id)::jsonb as user_id, -- D
json_object_agg(skill_title, skills -> skill_title -> 'rating')::jsonb as skills
FROM (
SELECT
user_id,
json_object_keys(skills) as skill_title, -- C
skills
FROM (
SELECT
(datasets -> 'user_id')::text as user_id,
datasets -> 'data' -> 'skills' as skills -- B
FROM (
SELECT
json_array_elements(json) as datasets -- A
FROM (
SELECT '/* the JSON data; see db<>fiddle */'::json
)s
)s
)s
)s
GROUP BY user_id
ORDER BY user_id
)s
A Make all array elements ({user_id: '42', data: {...}}) one row each
B First column safe the user_id. The cast to text ist necessary for the GROUP BY later which cannot group JSON output. For the second column extract the skills data of the user
C Extract the skill titles for using them as keys in (D.1).
D.1 skills -> skill_title -> 'rating' extracts the rating value from each skill
D.2 json_object_agg aggregates the skill_titles and each corresponding rating value into one JSON object; grouped by the user_id
D.3 json_build_object makes the user_id a JSON object again
E.1 user_id || skills aggregates the two json object into one
E.2 jsonb_agg aggregates these json objects into an array
E.3 jsonb_pretty makes the result looking pretty.
Result:
[{
"skill_1": 4,
"skill_2": 2,
"skill_3": 5,
"user_id": "123"
},
{
"skill_1": 1,
"skill_2": 3,
"skill_3": 4,
"skill_4": 42,
"user_id": "456"
}]