PostgreSQL: Extract specific information from json - sql

Question

I'm using PostgreSQL 9.6 and I have the following table, let's call it table1:
id | json_data
____________________
200 | {"state": [3, 4, 5]}
I want to be able to perform the following query: extract the array inside the "state" key in json_data for the record with id 1, but also remove some of the integers from the array in the process.
For example (in pseudo-code):
extract_state(id = 200, remove_numbers_from_json_data_state = [3, 5]) should return [4]

Answer 1

There is no built-in function for that, but you can easily write your own:
create function remove_numbers(p_array jsonb, p_nr variadic int[] )
returns jsonb
as
$$
select jsonb_agg(x)
from jsonb_array_elements(p_array) as t(x)
where t.x::int <> ALL(p_nr);
$$
language sql
immutable;
Then you can use it like this:
select id, remove_numbers(json_data -> 'state', 4,5)
from t1
where id = 1;
If you prefer to pass a JSON array value, you can define the function like this:
create function remove_numbers(p_array jsonb, p_to_remove jsonb)
returns jsonb
as
$$
select jsonb_agg(x)
from jsonb_array_elements(p_array) as t(x)
where t.x not in (select *
from jsonb_array_elements(p_to_remove))
$$
language sql;
Then you would need to use remove_numbers(json_data -> 'state', '[4,5]')

Answer 2

You'd use a query like this:
SELECT json_data->'state'->>1 FROM table1 WHERE id = 200;
You can test this way, without having a table:
SELECT '{"state": [3, 4, 5]}'::jsonb->'state'->>1
Here is a reference for using JSON in Postgres:
https://www.postgresql.org/docs/current/functions-json.html
It returns 4, not [4], so if you need to, you could select it as an array:
SELECT ARRAY[('{"state": [3, 4, 5]}'::jsonb->'state'->>1)::int]