Postgres SQL query, that will group fields in nested JSON objects - postgresql

Question

I need a SQL query in Postgres that produce a JSON with grouped/inherited data,
see example below.
having a table "issues" with following example data:
+--------------------------------------+-------+------------+-----------------------+
| product_id | level | typology | comment |
+--------------------------------------+-------+------------+-----------------------+
| e1227f18-0c1f-4ebb-8cbf-a09c74ba14f5 | 1 | electronic | LED broken |
| e1227f18-0c1f-4ebb-8cbf-a09c74ba14f5 | 1 | mechanical | missing gear |
| e1227f18-0c1f-4ebb-8cbf-a09c74ba14f5 | 1 | mechanical | cover damaged |
| e1227f18-0c1f-4ebb-8cbf-a09c74ba14f5 | 2 | electric | switch wrong color |
| e1227f18-0c1f-4ebb-8cbf-a09c74ba14f5 | 2 | mechanical | missing o-ring |
| e1227f18-0c1f-4ebb-8cbf-a09c74ba14f5 | 2 | electric | plug wrong type |
| 3567ae01-c7b3-4cd7-9e4f-85730aab89ee | 1 | mechanical | gear wrong dimensions |
+--------------------------------------+-------+------------+-----------------------+
product_id, typology and comment are string.
level is an integer.
I want to obtain this JSON:
{
"e1227f18-0c1f-4ebb-8cbf-a09c74ba14f5": {
"1": {
"electronic": [ "LED broken" ],
"mechanical": [ "missing gear", "cover damaged"]
},
"2": {
"electronic": [ "switch wrong color", "plug wrong type" ],
"mechanical": [ "missing o-ring" ]
}
},
"3567ae01-c7b3-4cd7-9e4f-85730aab89ee": {
"1": {
"mechanical": [ "gear wrong dimensions"]
}
}
}
So I begun to wrote a query like this:
SELECT array_to_json(array_agg(json_build_object(
product_id, json_build_object(
level, json_build_object(
typology, comment
)
)
))) FROM issues
but I didn't realize ho to group/aggregate to obtain the wanted JSON

Answer 1

step-by-step demo:db<>fiddle
SELECT
jsonb_object_agg(key, value)
FROM (
SELECT
jsonb_build_object(product_id, jsonb_object_agg(key, value)) as products
FROM (
SELECT
product_id,
jsonb_build_object(level, jsonb_object_agg(key, value)) AS level
FROM (
SELECT
product_id,
level,
jsonb_build_object(typology, jsonb_agg(comment)) AS typology
FROM
issues
GROUP BY product_id, level, typology
) s,
jsonb_each(typology)
GROUP BY product_id, level
) s,
jsonb_each(level)
GROUP BY product_id
) s,
jsonb_each(products)
jsonb_agg() aggregates some values into one JSON array. This has been done with the comments.
After that there is a more complicated step. To aggregate two different JSON objects into one object, you need to do this:
simplified demo:db<>fiddle
First you need to expand the elements into a key and a value column using jsonb_each(). Now you are able to aggregate these two columns using the aggregate function jsonb_object_agg(). See also
This is why the following steps look somewhat difficult. Every level of aggregation (level and product_id) need these steps because you want to merge the elements into single non-array JSON objects.
Because every single aggregation needs separate GROUP BY clauses, every step is done in its own subquery.