Postgres 14

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create table facts_themes(Categories_A  text[], Categories_B text[]);
insert into  facts_themes
VALUES
  ('{"A"}', '{"B","F","C"}'),
  ('{"B","A"}', '{"Z","B","F"}'),
  ('{"B","F"}', '{"A","E","R"}');
3 rows affected

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;
select categories_a, categories_b, array_except(categories_a, categories_b)
from facts_themes;


categories_a
categories_b
array_except


{A}
{B,F,C}
{A}

{B,A}
{Z,B,F}
{A}

{B,F}
{A,E,R}
{B,F}