Boolean search refers to a
search for
documents using
boolean operators to specify
what words the retrieved documents should contain.
Internet search engines that offer boolean
searching capability include
AltaVista,
Lycos,
Excite,
Northern Light, and
Inktomi-based search portals such as
HotBot and
Snap. Most of these require that you go to the "Advanced Search" part of
their site in order to use boolean search.
Boolean search isn't restricted to Internet search engines -- it's also a standard in many document retrieval systems.
Operators
Boolean logic includes three operators:
In the standard
syntax used by most boolean search engines, these operators are used
in
infix notation, meaning they are inserted between two words.
AND: This operator is used when you want to find documents that include all the words
you use in your search. For example,
dogs AND cats
will retrieve documents that include both the word "dogs" and the word "cats".
dogs AND cats AND parrots
will only retrieve documents that mention all three words.
OR: This operator is used when you want to find documents that mention at least one of the words\
.
dogs OR cats OR parrots
will retrieve any document that includes the word "dogs" as well as any document that includes the word \
"cats" or the word "parrots", without requiring that they include all three.
NOT: This operator is used when you want to exclude from your search results documents that
mention a particular word.
dogs NOT cats
will retrieve documents which include the word "dogs", but will omit any of them that include the word "\
cats".
Extensions
Some search engines, namely Alta Vista and Lycos, support the "NEAR" operator. This is not strictly
a boolean operator, but can be used in expressions along with the other boolean operators.
This operator allows you to specify that the retrieved documents should have the two related words
in close
proximity within the document. For example,
dean NEAR martin
retrieves documents that contain both words, and where they are close together. The exact proximity
varies from search engine to search engine, and is in some cases specifiable.
However, almost all the search engines allow the use of quotes to match exact phrases, so for the
example above, it would be better to search for "dean martin".
Complex expressions
It is possible to create boolean search expressions that combine multiple operators. When you do
this, it is often necessary to use
parentheses to say how the complex expression should be
interpreted. For example,
(dogs OR cats) AND veterinarian
will give different results than
dogs OR (cats AND veterinarian)
The first example will find documents that either mention dogs and veterinarians, or they
mention cats and veterinarians. The second will return a set of documents, some of which mention
dogs (maybe only dogs), and some of which mention both cats and veterinarians.