1. A method, comprising: receiving a database query for a database system that comprises a first database, a second database and a virtual relationship table, wherein the second database is a graph database and the first database is a different type of database than the second database; querying the virtual relationship table to determine whether the database query is requesting a relationship that is explicitly represented in the virtual relationship table; in response to determining that the database query is requesting a relationship that is explicitly represented in the virtual relationship table, using primary keys in the virtual relationship table to query for corresponding nodes in the graph database; in response to determining that the database query is requesting a relationship that is not explicitly represented in the virtual relationship table, querying the graph database to determine whether the relationship is implicitly represented in the graph database; retrieving responsive data from a memory device associated with the second database; and responding to the database query by providing the responsive data.