Boolean Searching

Learn about: How Boolean operators AND, OR and NOT can refine your search by combining or limiting terms.

Boolean logic is a system of showing relationships between sets by using the words "AND," "OR," and "NOT." (The term Boolean comes from the name of the man who invented this system, George Boole.) Boolean logic is recognized by many electronic searching tools as a way of defining a search string.

Boolean searching is an important tool that can be used when searching catalogs, indexes, online databases, and the web.

Here are some examples of boolean search strings:

  • mushing AND racing
  • caribou OR reindeer
  • fisheries NOT Alaska

Boolean logic is a bit more confusing than it seems. Sets get smaller the more "AND" is used, and larger the more "OR" is used. Although you might think "AND" would add more hits to a set, what it has actually added is limitations. The term "OR" adds possible options. A good way to illustrate how boolean logic works is through a Venn diagram. The circles in a Venn diagram illustrate different sets and the shaded areas show how the boolean terms form relationships between the sets.
boolean-and_venn.jpg

The blue shaded area represents all the hits in a database that contain works that are about both mushing and racing. The white parts of each circle represent those items that have information about mushing but not racing, or racing but not mushing. Only the items in the blue area will be retrieved. The boolean expression "AND" narrows a search.
boolean-or_venn.jpg

The set that contains hits on caribou or reindeer will not only contain those articles that contain information on both animals, but also those articles that are one, but not the other. All items will be retrieved. The boolean expression "OR" broadens a search.
boolean-not_venn.jpg

If you wanted to do research on fisheries outside of Alaska, you might try the above search. Using "not" will eliminate all hits that include information on both fisheries and Alaska. The boolean expression "NOT" narrows a search.

Adjusting Boolean terms to improve searching:

Finding the right number of hits can be tricky. Sometimes search tools will bring up thousands of hits, when all that is needed is one. Sometimes only a couple of hits emerge from a search when you need as many as possible. Sometimes you want just enough for a good two, ten or 20 page paper. How many is that? It depends on the context and on the quality or relevance of the hits. Chances are you will only use a fraction of the hits that emerge from a single search. In this sense, more is better. But you will still want to keep the number of hits to a size you can manage.

Broaden a search: mushing AND racing

Say the first example only generates two hits, and you want to read all you can on the topic -- how might you adjust the search string to generate more hits?

  1. Add synonyms or related terms with "OR"

    (mushing OR sled dogs) AND (racing OR competition)

  2. Eliminate terms linked by "AND"

    Searching on the term "mushing" by itself may bring up more relevant hits, or it may bring up material on aspects of mushing other than racing. Searching on "racing" alone will bring up mostly irrelevant hits. Broadening your search may mean broadening the topic you are searching. You may want to think carefully whether or not this is what you want.

  3. Choose a different database

    Perhaps the database you are searching simply doesn't hold enough relevant data. Often times a subject specific database will have more relevant resources than a general database. Or it could work the other way around -- a scientific database may only bring up technical reports when more general interest materials are wanted.

Narrow a search: caribou OR reindeer

  1. Add limiting terms with "AND" or "NOT":

    (caribou OR reindeer) AND alaska NOT artwork

  2. Eliminate terms linked by "OR"

    A search on just caribou or just reindeer will, of course, bring a smaller set. Narrowing your search may mean narrowing the topic you are searching. You may want to think carefully whether or not this is what you want to do.

  3. Choose a different search tool

    Perhaps the database you are using contains too much general material. Using a more specialized database may eliminate the need to weed out material that is not relevant to your research.

Remember: Not all databases are the same!

Different library catalogs and indexes handle boolean expressions differently.

Different approaches to boolean searches:

  • Some databases support nesting (using long search strings with multiple boolean terms and parenthesis), some do not.
  • Some databases will always read a search string left to right.
  • Some databases will always read boolean terms in one particular order; search terms linked by "not" may take precedence over terms linked by "AND" or "OR".
  • Some databases support special symbols such as: "&" for "AND", "+" for "OR" , "-" for "NOT."
  • Some databases use different rules for basic and advanced searches.
  • Some databases don't support boolean terms at all.

Read the HELP screens to clarify how to best express your search statement. Taking a few moments to read through the "how to search" instructions may save you hours of time and frustration.

This page was last modified on November 9, 2014