A relation represents a database link between two tables.
It can either be a 1:n-relation or an n:n-relation. Relations in MiningMart store the information
about foreign keys and primary keys as well as (optional) cross tables so that the
can use this information. Thus, relations can be parameters like concepts and features.
As such, they should belong to the conceptual level; however, since they also store
database-related information, they might also be said to belong to both levels (conceptual and relational).