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Research Help

Boolean Operators

Three simple words can have a big impact on your research. 

Boolean is a set of commands recognized by nearly every search engine, database, or library catalogue that:

  • help focus your search
  • connect various pieces of information

The three basic Boolean operators are:

  • AND
  • OR
  • NOT

Be sure to always capitalize Boolean commands. Some databases only recognize the operators when they're capitalized.

 

Using AND

Use AND in a search to:

  • narrow your results
  • tell the database that ALL search terms must be present in the resulting records

 

Example: dementia AND Alzheimer's

The dark blue section in the middle of the Venn diagram represents the result set for this search. The number of results in that section are smaller (or narrower) than they would be if searching each term on its own.

Venn diagram of Boolean search using dementia AND Alzheimer's. Overlapping area of circles illustrates search results.

 

Using 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: dementia OR Alzheimer's

Both circles represent the result set for this search. The number of results is larger (or broader) than it would be if searching each term on its own.

Venn diagram of Boolean search using dementia OR Alzheimer's. Overlapping area of circles illustrates search results.

 

Using NOT

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: dementia NOT Alzheimer's

The blue partial circle represent the result set for this search. The number of results is smaller (or narrower) than it would be if searching the term on its own.

Venn diagram of Boolean search using Alzheimer's NOT dementia. Blue circle with cut-out illustrates search results.

 

Search Order

Databases follow commands you enter and return results based on those commands. Be aware of the logical order in which words are connected when using Boolean operators: 

  • Databases usually recognize AND as the primary operator, and will connect concepts with AND together first.
  • If you use a combination of AND and OR operators in a search, enclose the words you want to separate with OR in parentheses.

Examples:

  • elderly AND (dementia OR Alzheimer's)
  • (elderly OR aged) AND (dementia OR Alzheimer's)

Adapted and used with permission from Southern Methodist University Library.

Tricks to Finding Good Results

Wildcard ?:

Find different spellings for search terms. Colo?r will give you results with color and colour.

Truncation *:

Find words with varied endings. Educat* will give you results with educate, educated, educator, education, educational, educates, etc.

Phrase Search " ":

Find the exact phrase, with your words in the exact same order as you entered them in the search. "Medication errors" will give you results with both of those words in direct order, instead of results with the words found separately.