Legitimately collected and accessed data must also be used appropriately according to laws, guidelines, policies or the (current) preferences of data subjects. For example, inconsistency between the data collection purpose and the data usage purpose may conflict with some privacy principles. In this contribution we motivate adopting the usage control model when joining vertically-separated relational datasets and characterize it as obligations within the Usage Control (UCON) model. Such obligations are defined by the state of the object (i.e., a dataset) in the UCON model with respect to the state of another object/dataset. In case of the join operation, dependency on two UCON objects (i.e., two datasets) results in a new type of UCON obligations. We describe also a number of mechanisms to realize the identified concept in database management systems. To this end, we also provide some example methods for determining whether two given datasets can be joined.