PostgreSQL: Identify missing value when comparing nested arrays

I have the following nested data in two columns:

Categories_A    Categories_B
{"A"}           {"B","F","C"}
{"B","A"}       {"Z","B","F"}
{"B","F"}       {"A","E","R"}

I would like to return in a new column the missing category/categories in categories_B when compared to categories_A – what is in Categories_A that is not in categories_B. So ideally:

Categories_A    Categories_B      Missing_Category
{"A"}           {"B","F","C"}     {"A"}
{"B","A"}       {"Z","B","F"}     {"A"}
{"B","F"}       {"A","E","R"}     {"B","F"}

So far what I managed is to return if there is a complete match between the columns:

select Categories_A, Categories_B,
    case
        when Categories_A = Categories_B then 'TRUE'
        else 'FALSE'
    end is_a_match
from facts_themes

Categories_A    Categories_B      is_a_match
{"A"}           {"B","F","C"}     FALSE
{"B","A"}       {"Z","B","F"}     FALSE
{"B","F"}       {"A","E","R"}     FALSE

>Solution :

There is no built-in function for that, but it’s easy to write your own:

create function array_except(p_one anyarray, p_two anyarray)
  returns anyarray
as
$$
  select array_agg(e)
  from (
    select e
    from unnest(p_one) as p1(e)
    except
    select e
    from unnest(p_two) as p2(e)
  ) x
$$
language sql
immutable
;

Then you can use it like this:

select categories_a, categories_b, 
       array_except(categories_a, categories_b) as missing_categories
from facts_themes

Online example