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This is an associative table with the additional metadata of an external ID that defines there relationship for lookup. It is possible for this to exist without an associated user, as when we import external IDs to validate them when accounts are created. It's also possible to have the association without an external ID (Table might be better called SubStudyAccounts). Users can be in more than one sub-study.
- accountId [FK] - optional
- studyId [FK] - required
- subStudyId [FK] - required
- externalId - optional. developers/admins wouldn't have one
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