A new family of Intensional RDBs (IRDBs), introduced in [1], extends the traditional RDBs with the Big Data and flexible and 'Open schema' features, able to preserve the user-defined relational database schemas and all preexisting user's applications containing the SQL statements for a deployment of such a relational data. The standard RDB data is parsed into an internal vector key/value relation, so that we obtain a column representation of data used in Big Data applications, covering the key/value and column-based Big Data applications as well, into a unifying RDB framework. Such an IRDB architecture is adequate for the massive migrations from the existing slow RDBMSs into this new family of fast IRDBMSs by offering a Big Data and new flexible schema features as well. Here we present the interoperability features of the IRDBs by permitting the queries also over the internal vector relations created by parsing of each federated database in a given Multidatabase system. We show that the SchemaLog with the second-order syntax and ad hoc Logic Programming and its querying fragment can be embedded into the standard SQL IRDBMSs, so that we obtain a full interoperabilty features of IRDBs by using only the standard relational SQL for querying both data and meta-data.