Boolean Searching on the Internet

Most library database searching. is based on the principles of Boolean logic. Boolean logic refers to the logical relationship among search terms, and is named for the British mathematician George Boole.

Boolean logic consists of three logical operators:

• OR
• AND
• NOT

Each operator can be visually described by using Venn diagrams, as shown below.

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OR

college OR university

Query:    I would like information about college.

• In this search, we will retrieve records in which AT LEAST ONE of the search terms is present.

• This is illustrated by:
  • the shaded circle with the word college representing all the records that contain the word "college"

  • the shaded circle with the word university representing all the records that contain the word "university"

  • the shaded overlap area representing all the records that contain both "college" and "university"

OR logic is most commonly used to search for synonymous terms or concepts.

Here is an example of how OR logic works:

Search terms	Results
college	17,320,770
university	33,685,205
college OR university	33,702,660

OR logic collates the results to retrieve all the unique records containing one term, the other, or both.

The more terms or concepts we combine in a search with OR logic, the more records we will retrieve.

For example:

Search terms	Results
college	17,320,770
university	33,685,205
college OR university	33,702,660
college OR university OR campus	33,703,082

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AND

poverty AND crime

Query:    I'm interested in the relationship between poverty and crime.

• In this search, we retrieve records in which BOTH of the search terms are present

• This is illustrated by the shaded area overlapping the two circles representing all the records that contain both the word "poverty" and the word "crime"

• Notice how we do not retrieve any records with only "poverty" or only "crime"

Here is an example of how AND logic works:

Search terms	Results
poverty	783,447
crime	2,962,165
poverty AND crime	1,677

The more terms or concepts we combine in a search with AND logic, the fewer records we will retrieve.

For example:

Search terms	Results
poverty	783,447
crime	2,962,165
poverty AND crime	1,677
poverty AND crime AND gender	76

A few Internet search engines make use of the proximity operator NEAR. A proximity operator determines the closeness of terms within a source document. NEAR is a restrictive AND. The closeness of the search terms is determined by the particular search engine. For example:

• NEAR in AltaVista (Power Search) is 10 words
• NEAR in Lycos (Lycos Pro) is 25 words

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NOT

cats NOT dogs

Query:    I want to see information about cats, but I want to avoid seeing anything about dogs.

• In this search, we retrieve records in which ONLY ONE of the terms is present

• This is illustrated by the shaded area with the word cats representing all the records containing the word "cats"

• No records are retrieved in which the word "dogs" appears, even if the word "cats" appears there too

Here is an example of how NOT logic works:

Search terms	Results
cats	3,651,252
dogs	4,556,515
cats NOT dogs	81,497

NOT logic excludes records from your search results. Be careful when you use NOT: the term you do want may be present in an important way in documents that also contain the word you wish to avoid.

 

Examples:

Query:    I need information about cats.

Boolean logic:    OR

Search:    cats OR felines

Query:    I'm interested in dyslexia in adults.

Boolean logic:    AND

Search:    dyslexia AND adults

Query:    I'm interested in radiation, but not nuclear radiation.

Boolean logic:    NOT

Search:    radiation NOT nuclear

Query:    I want to learn about cat behavior.

Boolean logic:    OR, AND

Search:    (cats OR felines) AND behavior

Note: Use of parentheses in this search is known as forcing the order of processing. In this case, we surround the OR words with parentheses so that the search engine will first process this part of the search. Next, the search engine with combine this result with the last part of the search. Using this method, we are assured that the OR terms are kept together as a logical unit.