Keyword search is known as an attractive alternative for structured query languages
in querying over graph-structured data. A keyword query is expressed by a set of keywords and
respond by a set of connected structures from the database, which totally or partially cover the
queried keywords. These results show how the queried keywords are related in the database.
Since there may be numerous results to a given query, a ranking function is essential to present
top-k more relevant results to the user. The effectiveness of this function directly affected the
effectiveness of the keyword search system. In this paper, we survey the proposed ranking
functions in the context of keyword search. First, the proposed models for the results of a
keyword query are discussed and a categorization of them is presented. Next, the effective
factors in determining the relevance of results are examined. Then, various ranking functions
for ordering the results of a query are described and categorized based on their main view in
determining the semantic of the results. Finally, we present an analysis of these classes and
discuss the evolution of new research strategies to resolve the issues associated with the
ranking of results in the keyword search domain.