Ranking the query result is a key requirement for keyword search in order to rank and make appear the most relevant results first. XML keyword search queries are different from HTML keyword search queries in the way query results are ranked. Normally, documents are ranked by HTML search engines (such as Google) based (partly) on their hyperlinked structure (Brin and Page, 1998; Kleinberg, 1999). XML keyword search queries can return nested elements. Hence, ranking has to be computed at the granularity of XML elements, as opposed to entire XML documents. Since the semantics of containment links (relating parent and child elements) is very different from that of hyperlinks, computation of rankings at the granularity of elements is complicated. As a result, ranking techniques which are used for computation solely based on hyperlinks (Brin and Page, 1998; Kleinberg, 1999) cannot directly be applied for nested XML elements. Some of the works on result rankings for XML keyword query results include XRANK (Guo et al., 2003), XSEarch (Cohen et al., 2003), EASE (Li et al., 2008) and XReal (Bao et al., 2010a).
The rest of the chapter is organized as follows. The related works based on tree data model and digraph data model are reviewed in Section 2.2 and 2.3 respectively. Subsequently, the works done on the result ranking are reviewed in Section 2.4. Also, other related works in XML keyword search are reviewed in Section 2.5. In Section 2.6, the approaches utilizing statistics of