1. A computer-implemented method of creating associations between search queries and items, the method comprising: detecting a behavior-based association between a search query and a first item, said behavior-based association based at least partly on item selection actions of users who have submitted the search query, said behavior-based association reflecting a tendency of users who submit the search query to select the first item; detecting a substitutability association between the first item and a second item, said substitutability association based at least partly on an analysis of content of the first and second items; and creating, in computer storage, a mapping that associates the search query with the second item, said mapping created based at least partly on both (1) the detected behavior-based association between the search query and the first item, and (2) the detected substitutability association between the first item and the second item; said method performed in its entirety by a computer system that comprises one or more computers.