Major online database companies like ProQuest and EBSCOhost offer many disparate databases; you can, however, search all of their databases at once. This technique also works for some major journal and e-book publishers.
Search platform for over 65 databases, from Academic Search Ultimate to The Serials Directory. To search all databases at once, choose the Select All box, which may be found within one of EBSCO's databases, above the top search box.
Search platform for 9 databases, including the New York Times, Wall Street Journal, Dissertations and Theses A & I, and ComDisDome. Includes some scholarly articles, as well as newspapers and dissertations.
Provides access to a large bibliographic database of scientific and medical publications of the Dutch publisher Elsevier.
Comprehensive online collection of scientific, technological and medical journals, books and reference works.
While searching multiple databases at once can be useful, searches default to the lowest-common denominator.
This means that all the wonderful advanced features available in each database are only available when searching multiple databases--if all of the databases contain that field, limiter, feature, etc.
Thus, few of the advanced search techniques available in individual databases will be available in a multiple database search. Some pitfalls to watch out for: