No need to bother any relationships. Each concept has a domain. So, your J-code concepts
2718872 J9201 Injection, gemcitabine hydrochloride, 200 mg
2718921 J999 Not otherwise classified, antineoplastic drugs
are both domain “Drug”, and hence should be recorded in DRUG_EXPOSURE.
The first one is mapped to an RxNorm concept, which should be used in drug_concept_id. The second one is not. Instead, it is a standard concept by itself and can go right into drug_concept_id. The reason is that “antineoplastic drug” is not a specific ingredient, and therefore cannot be mapped to RxNorm.