Currently, Web database systems are widely used to construct Web sites for their excellent capability of providing on-line information. In this paper, we analyse the workflow of a Web database system dealing with a Web page request. After forwarding the data request to database servers, Web servers must wait until all or part of query result is sent back by the database server before proceeding to next step. This is a typical serial queuing system, which is not easy to model except the service-time distribution is exponential. We have classified the different cases and given an approximate method to model a Web database system. As the service time of database servers is a primary factor in determining the input characteristics of Web servers, it is very important to investigate the relationship between database servers and Web servers.