Radio Frequency Identification (RFID) systems employ pervasive, low-cost devices to identify physical objects and track item movements. The integration of sensors into RFID tags has recently generated a lot of interest among the RFID community. Integrating automatic identification and sensors at the RFID tag hardware level not only provides an identity to an object but also provides information regarding the conditions of the object carrying the tag. With the capacity of collecting real-time tag data as empowered by the tags, an RFID system can be viewed as a virtual database. Users can query this virtual database for the data they need just as they are interacting with any traditional persistent database. In this paper, we present a framework to virtualize an RFID system as a database and provide mechanisms to implement the high-level user queries. We discuss both the data model and system model of the virtual database. As security and privacy are the main concerns for RFID systems, our framework also ensures data confidentiality and integrity in user queries.