Keyword Search Over Relational Databases (KSORD) enables casual or Web users easily access databases through free-form keyword queries. Improving the performance of KSORD systems is a critical issue in this area. In this paper, a new approach CLASCN (Classification, Leing And Selection of Candidate Network) is developed to efficiently perform top-k keyword queries in schema-graph-based online KSORD systems. In this approach, the Candidate Networks(CNs) from trained keyword queries or executed user queries are classified and stored in the databases, and top-k results from the CNs are leed for constructing CN Language Models (CNLMs). The CNLMs are used to compute the similarity scores between a new user query and the CNs from the query. The CNs with relatively large similarity score, which are the most promising ones to produce top-k results, will be selected and performed. Currently, CLASCN is only applicable for past queries and New All-keyword-Used (NAU) queries which are frequently submitted queries. Extensive experiments also show the efficiency and effectiveness of our CLASCN approach.