How to implement map/array_walk on PostgreSQL?

Question

PostgreSQL

The task is this.
[{"a": 1, "b": 2}, {"a": 10, "b": 20}]
It is necessary to turn such JSON into
[1, 10]
such for each element of the array, extract the value with the key "a".
In JS/PHP this can be done in one line by converting the array elements. How can this be done in PostgreSQL?

While I was able to reach such a request
select jsonb_array_elements(my_column)->>'a'

. It returns a set of the necessary values, but in the form of several lines. And you need to get them in the form of a single array.

Answer 1

Well assemble back into an array

select array_agg(v)
  from (select jsonb_array_elements(col)->'a' v
    from (values ('[{"a": 1, "b": 2}, {"a": 10, "b": 20}]'::jsonb)) as t(col)
  ) t1