In addition, DISCOVER[25]
, BANKS[22]
, and
DBXplorer
[21]
are built on the top of relational data-
bases. DISCOVER and DBXplorer output trees of tu-
ples connected through primary-foreign key relation-
ships that contain all the keywords of the query, while
BANKS identifies connected trees in a labeled graph
using an approximation to the Steiner tree problem,
which is an NP-hard problem. More recently, Liu et
al.
[27]
proposed a ranking strategy to solve the effec-
tiveness problem for relational databases which em-
ploys phrase-based and concept-based models to im-
prove the search effectiveness. Kimefeld and Sagiv[26]
developed a keyword proximity search in the relational
databases which shows that the answer of keyword
proximity search can be enumerated in ranked order
with polynomial delay, under data complexity.