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Medical Sciences: Database Search Tips: Boolean Operators

Recommended resources for the College of Osteopathic Medicine


Boolean searching, i.e. using Boolean Operators in between your search words, is the single most effective 'advanced' search technique you can use to improve your search results, making them more relevant and precise. They connect your search words together to either narrow or broaden your set of results. The three basic boolean operators are: ANDOR, and NOT

Boolean operators that help you narrow your results:

  • Boolean AND
    • Instructs the search engine to only bring back results that contain BOTH search words
    • Databases usually recognize AND as the primary operator, and will connect concepts with AND together first
  • Boolean NOT
    • Instructs the search engine to NOT bring back any results that contain a specific search word
    • Take care when using this one - results containing the search terms you are interested in may be left out of your results if you are using NOT - simply because they also happened to contain the unwanted search term.

Boolean operators that help you broaden your results:

  • Boolean OR
    • Instructs the search engine to bring back results that contain either one search word OR the other - it will bring back results containing either word - so will usually end up bringing back more results.
    • This can be a good choice when your search topic word has several synonyms and you aren't sure which one will be used in the documents you seek.
    • Take care when using OR - sometime it is necessary to use parentheses in conjunction with OR in order to get the search to run correctly. (See the 'Parentheses/Nesting' tab above for more details.)


Use AND in a search to:

  • narrow your results
  • tell the database that ALL search terms must be present in the resulting records
  • example: cloning AND humans AND ethics

The purple triangle in the middle of the Venn diagram below represents the result set for this search. It is a small set using AND, the combination of all three search words.

Be aware:  In many, but not all, databases, the AND is implied. 

  • For example, Google automatically puts an AND in between your search terms.
  • Though all your search terms are included in the results, they may not be connected together in the way you want.
  • For example, this search:  college students test anxiety  is translated to:  college AND students AND test AND anxiety. The words may appear individually throughout the resulting records.
  • You can search using phrases to make your results more specific.
  • For example:  "college students" AND "test anxiety". This way, the phrases show up in the results as you expect them to be.

How to Perform Boolean Searches

From a single search bar:

  • Type your Boolean operator, in all caps, in the search box between your search terms.
  •  Some database search engines can't 'see' those operators unless they are capitalized.
  • Some databases automatically 'assume' a Boolean AND to be in between two or more search words.  But not all do, so it's safest to type the Boolean connectors in capital letters. ‚Äč

Use OR

Use OR in a search to:

  • connect two or more similar concepts (synonyms)
  • broaden your results, telling the database that ANY of your search terms can be present in the resulting records
  • example: cloning OR genetics OR reproduction

All three circles represent the result set for this search. It is a big set because any of those words are valid using the OR operator.


Use NOT in a search to:

  • exclude words from your search
  • narrow your search, telling the database to ignore concepts that may be implied by your search terms
  • example:  cloning NOT sheep

How to Perform Boolean Searches

From a database's advanced search interface:

  • Many databases offer several search boxes (and the ability to add more if needed) to accommodate all your search terms.
  • Here, to add a Boolean operator to your search, you simply choose the one you want from the pull-down list (such as in the image above) that is available in between each pair of search boxes.