Harbour extends the Clipper Replaceable Database Drivers (RDD) approach. It offers multiple RDDs such as [[DBASE|DBFdBase]] (DBF), DBFNTX, DBFCDX, DBFDBT and DBFFPT. In Harbour, multiple RDDs can be used in a singleone application, and new logical RDDs can be defined by combining other RDDs. The RDD architecture allows for [[Inheritance (object-oriented programming)|inheritance]], so that a given RDD may extend the functionalityfunctions of other existing RDD(s). Third-party RDDs, like RDDSQL, RDDSIX, RMDBFCDX, [[Advantage Database Server]], and Mediator exemplify some of the RDD architecture features. DBFNTX implementation has almost the same functionality of DBFCDX and RDDSIX. NETIO and LetoDB<ref>{{cite web |url=https://sourceforge.net/projects/letodb |title=LetoDB |publisher=Sourceforge.net |access-date=2013-12-09}}</ref> provide remote access over [[Transmission Control Protocol|TCP]] protocol(TCP).
Harbour also offers [[ODBC]] support by means of an [[Objectobject-oriented programming|OOP]] (OOP) syntax, and [[ActiveX Data Objects|ADO]] (ADO) support by means of [[OLE Automation|OLE]] (OLE). [[MySQL]], [[PostgreSQL]], [[SQLite]], [[Firebird (database server)|Firebird]], [[Oracle (database)|Oracle]] are examples of databases which Harbour can connect to.
[[xBase]] technologies often are confused often with [[RDBMSrelational database management system]] (RDBMS) software. AlthoughWhile thisxBase iswas promoted as truerelational, xBase is more than a simple database system, as at the same time, xBase languages using purely DBF can notcannot provide the full conceptfeatures of a realfull RDBMS.
