A distributed database is a collection of data sources distributed across many computers. It is one logical and centrally managed database stored in multiple physical locations. In order to access data locally and manage data globally, a good distributed database system (DDBS) that can supply reliable and up-to-date information is critical. A good DDBS should have the functionality such as providing users timely and flexible access to information, providing the personnel tools to analyze the data in a meaningful manner, and allowing the personnel to control the safety and integrity of the data. However, query processing in such a distributed database system is complicated. For example, how to keep data up-to-date when it is physically dispersed and how to propagate information to the whole DDBS when an update occurs are some important issues. In response to these issues, we propose an expert query system that uses Bayesian network as its framework to propagate information and to keep data up-to-date for a distributed database system. The goal of this query system is to assist the personnel in maintaining the data dynamically and to assist the users in accessing reliable and timely information.