Query to extract ids from a deeply nested json array object in Presto - sql

Question

I'm using Presto and trying to extract all 'id' from 'source'='dd' from a nested json structure as following.
{
"results": [
{
"docs": [
{
"id": "apple1",
"source": "dd"
},
{
"id": "apple2",
"source": "aa"
},
{
"id": "apple3",
"source": "dd"
}
],
"group": 99806
}
]
}
expected to extract the ids [apple1, apple3] into a column in Presto
I am wondering what is the right way to achieve this in Presto Query?

Answer 1

If your data has a regular structure as in the example you posted, you can use a combination of parsing the value as JSON, casting it to a structured SQL type (array/map/row) and the using array processing functions to filter, transform and extract the elements you want:
WITH data(value) AS (VALUES '{
"results": [
{
"docs": [
{
"id": "apple1",
"source": "dd"
},
{
"id": "apple2",
"source": "aa"
},
{
"id": "apple3",
"source": "dd"
}
],
"group": 99806
}
]
}'),
parsed(value) AS (
SELECT cast(json_parse(value) AS row(results array(row(docs array(row(id varchar, source varchar)), "group" bigint))))
FROM data
)
SELECT
transform( -- extract the id from the resulting docs
filter( -- filter docs with source = 'dd'
flatten( -- flatten all docs arrays into a single doc array
transform(value.results, r -> r.docs) -- extract the docs arrays from the result array
),
doc -> doc.source = 'dd'),
doc -> doc.id)
FROM parsed
The query above produces:
_col0
------------------
[apple1, apple3]
(1 row)