This has been discussed en detail (e.g. here and here and here). We do have a similar two-tier logic in DRUG_EXPOSURE and DRUG_ERA, and likewise in CONDITION_OCCURRENCE and CONDITION_ERA. In contrast to those, in the Visit situation the higher-level VISIT_OCCURRENCE table needn’t be constructed from the VISIT_DETAIL. It is up to the ETLer to figure it out, and whether a VISIT_DETAIL is even needed. Usually, this is only the case for hospitalizations.