Sundarapandian et al. (Eds) : ACITY, AIAA, CNSA, DPPR, NeCoM, WeST, DMS, P2PTM, VLSI - 2013 pp. 147–154, 2013. © CS & IT-CSCP 2013 DOI : 10.5121/csit.2013.3415         Ahmed Elsayed 1 , Ahmed Sharaf Eldin 1 , Doaa S. El Zanfaly 1, 2 1 Information Systems Department, Faculty of Computers and Information, Helwan University, Cairo, Egypt eng_ahmedyakoup@yahoo.com, profase2000@yahoo.com, doasad71@yahoo.com 2 Informatics and Computer Scinence, British University in Egypt, Cairo, Egypt doaa.elzanfaly@bue.edu.eg ABSTRACT Keyword Search Over Relational Databases (KSORDB) provides an easy way for casual users to access relational databases using a set of keywords. Although much research has been done and several prototypes have been developed recently, most of this research implements exact (also called syntactic or keyword) match. So, if there is a vocabulary mismatch, the user cannot get an answer although the database may contain relevant data. In this paper we propose a system that overcomes this issue. Our system extends existing schema-free KSORDB systems with semantic match features. So, if there were no or very few answers, our system exploits domain ontology to progressively return related terms that can be used to retrieve more relevant answers to user. 1. INTRODUCTION A significant amount of plain text and structured data has been stored side by side in relational databases for decades. In order to query this data, users have to know the database schema and then use Structure Query Language (SQL) to issue a precise, unambiguous and well-formed query. To relieve users from doing this, Keyword Search over Relational Database (KSORDB) enables casual users to query this data using a set of keywords called keyword query. Existing KSORDB approaches can be categorized into two main categories: Schema-based approaches [1-3] and Schema-free approach [4-7]. In order to process a keyword query, the schema-based approach uses the schema graph to generate Candidate Network (CNs) and then evaluate these networks using SQL queries. While in the second approach, schema-free, the database is modeled as a data graph where nodes are tuples and edges are foreign - primary key relationships. The graph is then traversed, at the query time, to answer keyword queries. The main difficulties with the schema-based KSORDB systems are in generating optimal SQL queries from a huge number of CNs. Moreover, the generated queries usually contain many join