against the underlying, sometimes complex, database
schemas. These traditional querying methods are pow-
erful but unfriendly to non-expert users. Keyword
searches are an alternative means for querying data-
bases, which are simple and yet familiar to most inter-
net users since they only require the input of some
keywords. While keyword searches have proven effec-
tive for text documents (e.g., hypertext markup lan-
guage (HTML) documents), the problem of keyword
searches on structured data (e.g., relational databases)
or the semi-structured data (e.g., XML databases) is
not straightforward and well studied.