In public I have a table called "decks". Now I am trying to create another table there called "cards" that references "decks":

create table cards (
  id uuid default uuid_generate_v4(),
  deck_id uuid references public.decks.id on delete cascade,
  created_at timestamp with time zone default CURRENT_DATE,
  front text not null,
  back text not null,
  primary key (id)
);

However when I run this query, supabase tells me:

cross-database references are not implemented: "public.decks.id"

What does this mean?

You only need the table name, you don't need to specify the id field.

oh, but how does it relate the 2 tables to each other?

By the table's primary key

ah got it