Best practice: Record associated to one or all

Hi

What is the best practice to store records which are either related to one parent OR to all parents?

Related to one is clear (set parent_id to id), but which method is the better approach:

Set parent_id to 0 to make the record available to all parents or set it to "null" and make a "global switch" (tinyint 0/1)?

In real life a document can belong to one company OR (not at the same time) to all companies.

What would you do?

Thanks,

Frank

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