Large-scale application systems usually consist of various databases for different purposes. However, the increasing use of different databases, especially NoSQL databases, makes it increasingly challenging to use and maintain such systems. In this paper, we demonstrate a framework for designing a foreign data wrapper (FDW) for external data sources. We propose a novel method to access heterogeneous databases, including SQL and NoSQL databases, by using a unified access layer. This method was applied in some real business applications of Alibaba, in which we were able to do various operations on Redis, MongoDB, HBase, and MySQL by using a simple SQL statement. In addition, the information exchange and data migration between these databases can be done by using unified SQL statements. The experiments show that our method can maintain good database performance and provide users with a lot more convenience and efficiency.