Drill 23: Rewrite it — The relational model and psql

create table courses (
  slug text primary key,
  title text not null,
  published boolean not null default false
);

create table lessons (
  id bigint generated always as identity primary key,
  course_slug text not null references courses(slug) on delete cascade,
  slug text not null,
  title text not null,
  minutes integer not null check (minutes > 0),
  unique (course_slug, slug)
);