Why from_id, to_id, and type_id do not have the same type and default values?

[Konard]

https://github.com/deepcase/deeplinks/blob/94d5926fdfaa0003c4a9c28980840d66512fdc4f/migrations/1616701513782-links.ts#L19

CREATE TABLE ${SCHEMA}."${TABLE_NAME}" (
  id bigint PRIMARY KEY,
  from_id bigint DEFAULT 0,
  to_id bigint DEFAULT 0,
  type_id bigint NOT NULL);

As far as I understand at the moment, from_id and to_id are nullable, but have 0 as a default value.
Type is not nullable, but it has no default value.

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Forked from deep-foundation/deeplinks#15 by https://github.com/konard/gh-org-migrator