The omop_relationship table provides a reference list of all types of relationships that can be used to associate any two concepts in the omop_concept_relationship table.
Further details can be found in the OMOP CDM Vocabulary Tables specifications - https://ohdsi.github.io/CommonDataModel/cdm53.html#Vocabulary_Tables

Due to some data quality issues identified by research groups who have been using the returned OMOP derivation dataset, the relevant record-level tables are now restricted. Please see Field 20142 for more information.

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