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A large amount of unstructured data are being continuously generated by mobile applications, e.g., Facebook, Twitter and Foursquare, due to the popularity of smart devices, e.g., iPad, iWatch and smartphone. These data are undoubtedly of great value but need to be first well stored and then analyzed. Nevertheless, traditional relational database management system (RDBMS) is not well qualified for this job. As a result, NoSQL databases are proposed and become very popular. Judging from the present situations, RDMBSs play a better role in managing relational data, and thus application developers have to face the situation of co-existence of both RDBMS and NoSQL databases, and knowing the details of the underlying data distributions and structures of different databases. To this end, in this paper, we aim to propose a unified, transparent query engine, as a middleware, called "ZQL", by using MySQL and Hive as underlying two exemplary databases, however its usage goes far beyond these two databases. ZQL aims to support application development by hiding the specific details of both NoSQL databases and RDBMS. Finally, extensive experimental results show the effectiveness and functionalities of the proposed and implemented ZQL middleware.