Get unique nested JSON keys with JQ

Question

How to get the unique keys from attributes key with JQ

{"id":1, "attributes":{"a": 1, "b": 2, "c": 3}}
{"id":2, "attributes":{"a": 4, "b": 5, "d": 6}}
{"id":3, "name":"ABC"}

Result like this [ "a", "b", "c", "d" ]

I'm try like this

jq '.attributes' test.json | jq -r '[inputs | keys[]] | unique | sort'

or

jq -r '[inputs.attributes | keys[]] | unique | sort' test.json

but getting error

jq: error (at:11): null (null) has no keys

Answer 1

One way could be using reduce on subsequent inputs :

jq 'reduce inputs.attributes as $a (.attributes; . + $a) | keys'

[
  "a",
  "b",
  "c",
  "d"
]

Demo

Answer 2

Along the lines of your second attempt:

jq -n '[inputs.attributes // empty | keys_unsorted[]] | unique'

The important point is that we have to take care of the case where there is no "attributes" key.

Note also that unique sorts, so (unless you're using gojq) we can use keys_unsorted to avoid redundant sorting.

Answer 3

With slurp:

jq -s 'map(.attributes|keys?)|add|unique' test.json