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

mushing AND racing
The blue shaded area above represents the hits in a database that contain information about mushing AND racing. The white part of the circles represent 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 operator AND narrows a search.


boolean-or_venn.jpg

caribou OR reindeer
The blue shaded area above represents the hits in a database that contain information about caribou OR reindeer. This search results in hits containing information about either caribou or reindeer. The Boolean operator OR broadens a search.


boolean-not_venn.jpg

fisheries NOT Alaska
If you wanted to do research on fisheries outside of Alaska, you might try the above search. The Boolean operator NOT eliminates all hits containing a specific word, so be careful using the NOT Boolean operator.  The Boolean operator NOT excludes words from 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.

Narrow a search: mushing AND racing

  1. Search for different concepts by using AND

    Searching for the term mushing by itself may bring up relevant hits, but it may retrieve too many hits. Narrow your search by adding another concept. For example, mushing AND racing will retrieve information containing both terms, not just one term.

    Let's take a look at this in Google. We'll first search for the term mushing, which brings up approximately 1,260,000 results.
    A search for mushing brings up 1,260,000

    Now, narrow the search by adding another concept using the AND Boolean operator. (Note, Google automatically ANDs terms. In Google you don't need to type the word AND, but we did for demonstrative purposes.) The second search results in fewer hits. This is because AND narrows a search.
    Search for mushing AND racing
     

  2. Still not finding relevant hits? Select 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.

Broaden a search: caribou OR reindeer

  1. Search for synonyms and related terms by using OR
    Searching for the term caribou by itself may bring up relevant hits, but it may not retrieve articles that use the term reindeer in place of caribou. Broaden your query by using OR to search for synonyms and related terms.

    Let's take a look at this in Google. We'll first search for the term caribou, which brings up approximately 16,200,000 results.
    Search for caribou

    Now, broaden the search by using OR to search for reindeer, a related term of caribou. The second search results in more hits. This is because OR broadens a search.
    search for caribou OR reindeer results in more hits than just  searching for caribou
     

  2. Still not finding relevant hits? Select a different database.

    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.

Create a complex search by using AND and OR

Sometimes you'll need to create a complex search that uses AND as well as OR. For example, (caribou OR reindeer) AND Alaska. Notice this search retrieved fewer hits than the (caribou OR reindeer) search because AND narrows a search.

Complex search using AND and OR
 

 

Different library catalogs and databases 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.

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 October 12, 2016