InfoSkills for Humanities and Digital Sciences

What are Boolean operators?

Boolean operators make it possible to combine search terms in a search. They are named after George Boole, a 19th-century British mathematician who invented Boolean algebra, the mathematical system that underlies logic in computers. Boole's work laid many of the foundations for the digital revolution. 

There are three Boolean operators: AND, OR and NOT. Note that Boolean operators, when used in a database search, must be capitalized. This ensures the operators are identified as such and not ignored as common words.

AND

Use AND in a search to:

• narrow your results;
• tell the database to combine search terms so that each search result contains all of the terms. 
• Example: cats AND dogs

The overlapping yellow area in the so-called 'Venn diagram' on the right represents the results for this search. It's a small result set, containing results that are about both cats and dogs.

INOTEI
In many, but not all, library databases the Boolean operator AND is 'implied': the operator AND is automatically placed between adjacent words that are typed into the search box. For example, human rights is interpreted as human AND rights. 

OR 

Use OR in a search to:

• broaden your results;
• tell the database to combine the search terms so that each search result contains at least one of the terms. 
• Example: cats OR dogs

The contents of both circles represent the results for this search. It's a large result set because each result containing any of the search terms is found. 

NOT

Use NOT in a search to: 

• narrow your results;
• tell the database to exclude all terms that follow the operator from the search results.
• Example: cats NOT dogs

Again, the yellow area represents the results. Items about dogs as well as items about both cats and dogs are eliminated from the result set. 

Watch this video (2.15 mins) to see how database searches are broadened and narrowed using Boolean operators.