Drill 21: List the steps — Types, NULL and schema design

create table certificates (
  id text primary key,
  holder_name text not null,
  issued_at timestamptz not null default now(),
  score numeric(5,2) not null check (score between 0 and 100),
  expires_at timestamptz,
  metadata jsonb not null default '{}'::jsonb,
  constraint expiry_after_issue
    check (expires_at is null or expires_at > issued_at)
);