Reference

Boolean Operators are used to connect and define the relationship between your search terms.  When searching electronic databases, you can use Boolean Operators to either broaden or narrow your search results.  The three Boolean Operators are AND, OR and NOT.

Boolean Operators

Boolean operators are simple words (AND, OR and NOT) used as conjunctions to combine or exclude keywords in a search, resulting in more focused search results.

OR

• Broadens or expands your search
• Is used to retrieve like terms or synonyms
• Finds all items with either teenager OR adolescent
• In set theory and math, "union" is inclusive "OR".
  "OR" = teenager U adolescent

AND

• Narrows or limits your search
• Used to retrieve unrelated terms
• Finds items with both diet and children
• In set theory and math, "intersection" is "AND".
  "AND" = diet ∩ children

NOT

• Narrows or limits your search
• Finds the term "spider" not "monkey"
• Use the NOT operator with caution
• May eliminate relevant records

Note:
AND is the default or implied operator in Usearch, Google, Scopus, PubMed, EBSCOhost, and most search interfaces. 
"ecotourism sustainable" is the same as "ecotourism AND sustainable"

In Usearch, EBSCOhost, SCOPUS, and PubMed, Boolean operators (AND, OR, NOT) must be entered in upper case.

Phrase Searching

Phrase searching is using quotations.

For instance:

"international olympic committee"
"Utah tennis"

It finds the exact phrase, and items with words in the order typed.  One exception is Scopus.  Scopus uses curly brackets or braces for {exact phrase} searching.  In Scopus, quotes are used for "loose/approximate phrase" searching.

Truncation Stemming

Truncation or stemming is using an asterisk *.  It is also known as a wildcard.  Truncation is a symbol that retrieves all the suffixes or endings of a word.

For instance:

school*             retrieves school, schools, schooling, schooled, etc.
latin*                 retrieves latina, latino, latinx, latinos, latinas, latin, latinization, etc.

Note:
In the Library of Congress, % (percent sign) is a single character wildcard and ? (question mark) is truncation for multiple characters.

Nesting

Nesting is commonly used when combining more than one Boolean operator (OR, AND).  Most search interfaces search left to right.  Using parentheses in a search changes the order of operation.

For instance:

(moral* OR ethic*) AND (assisted suicide OR euthanasia)

(ski OR skis OR skiing OR snowboard*) AND video*

Proximity or Adjacency Operators

Proximity operators allow you to find one word within a certain distance of another.

With (w), Near (n), Next (n), or Pre (p) are common proximity operators.

Note:
Read the database help to see if proximity operators can be used in your searches.

Thanks to Alfred Mowdood for authoring these instructions.