Boolean Operators | Examples, Definition & List
Boolean operators are the words and symbols used to narrow or expand a search on a database. Examples of Boolean operators include “AND,” “OR,” and “NOT.” A Boolean search is therefore a search that uses Boolean operators.
Boolean operators are used together with keywords to form a Boolean string that makes your search more precise. This enables the database to identify the most pertinent sources.
Using Boolean operators
Boolean operators are a useful tool for identifying relevant sources, evaluating whether a research topic has a high search volume, and formulating unique and valuable research questions. Besides being used when conducting original research, Boolean operators are also helpful in preparing literature reviews and systematic reviews.
| Boolean operator | Function | Example |
|---|---|---|
| NOT or AND NOT | Search for sources that contain only the first keyword and not the second keyword | holiday NOT vacation |
| AND | Search for sources that contain both/all keywords | monochrome AND polychrome |
| OR | Search for sources that contain either keyword | tsunami OR typhoon |
| Asterisk * | Include results that contain a variation of the keyword | gift*
The results will include words such as “gift-giving” and “give.” |
| Quotation marks “” | Search for the exact phrase | “nuclear physics” |
| Parentheses () | Groups keywords and searches for keywords in order | (velvet OR silk) AND costuming |
AND
AND is used to search for all of the listed keywords. You can use “AND” to yield results containing two or more particular keywords.
OR
OR yields results containing at least one of the listed keywords.
You can use “OR” to search for multiple keywords at once where sources containing all keywords are not necessary.
NOT
NOT is used to search for results that include the first keyword but not the second keyword. When using this operator, be sure to place the keywords in the correct order according to what you want to search for (i.e., place the keyword you want to search for first).
You can use “NOT” when you only want to search for one specific keyword.
Parentheses: ()
Parentheses can be used to group keywords and search for them in a specific order. The keywords and Boolean operators that are within parentheses will be searched for first, and keywords outside parentheses will be searched for after this.
In the example below, the results will first include sources that contain “Psyche” or “Apollo,” and these will then be refined to include results that feature “gaze” as well.
Multiple sets of parentheses can be used. In this case, the search will first filter results for the innermost parenthetical keywords, followed by the outer parentheses and then the keywords that are outside parentheses.
Quotation marks: “”
Quotation marks are used to search for exact terms.
You can use quotation marks when you want to search only for a precise keyword or keywords.
Asterisk: *
Asterisks are used to include variations of the keyword in results.
You can use asterisks when you want to search both for your keyword and for derivative words.
This term will include results such as “realistic,” “realist,” and “reality.”
Different databases may use Boolean operators in different ways, so be sure to check the specifics for the search parameters of the database you are using.
Note: Databases and search engines often offer an “advanced search” tool that includes the most common Boolean operators in a list, as shown in the JSTOR example below.
Proximity operators
Proximity operators such as WITHIN (Wx), SENTENCE, and NEAR (Nx) filter results based on the proximity of two or more keywords.
| Proximity operator | Function | Example |
|---|---|---|
| SENTENCE | Searches for two or more keywords in the same sentence | Childhood SENTENCE bullying will yield results that include both “childhood” and “bullying” within the same sentence, such as “Bullying is particularly impactful when it occurs in childhood.” |
| WITHIN (Wx) | Searches for keywords that occur within x words of one another in the order listed | Pilgrimage W trail will yield results where “trail” occurs within five words of “pilgrimage.”
This search will yield results like “pilgrimage trail” as well as “pilgrimage routes on historical trails.” It will exclude results in which the order is reversed, such as “trails that originated as pilgrimage routes.” |
| NEAR (Nx) | Searches for keywords that occur within x words of each other regardless of the order listed | Pilgrimage N5 trail will yield results where “trail” occurs within five words of “pilgrimage,” such as “trails used for pilgrimage” or “pilgrimage trails.” |
Frequently asked questions about Boolean operators
- What is a Boolean search?
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A Boolean search uses Boolean operators (e.g., “AND,” “NOT”) and keywords to narrow or expand search results. You can use Boolean searches to:
- Search exact keywords
- Search keyword variations
- Prioritize keywords
- Exclude keywords
- What are proximity operators and how do I use them?
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Proximity operators, such as NEAR (Nx), WITHIN (Wx), and SENTENCE, can be used along with keywords to filter for results that include the keywords within a certain proximity to each other.
Different proximity operators are used to obtain different results. For example, Wx identifies sources where the keywords occur within a specific number of words (x) of each other and in the order listed.
- How do I search research databases efficiently?
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Make sure you have a clear idea of the parameters of your research and the key terms you want to search. Then choose a database that is relevant to your research (e.g., Cochrane, JSTOR, Medline).
Use specific keywords and their variations and synonyms when searching for scholarly sources.
Some databases have a “subject search” option, which can be useful. Become familiar with Boolean operators to efficiently combine keywords, search for exact phrases, and exclude specific terms.
