Internet Tutorials

Boolean Searching on the Internet

A Primer in Boolean Logic

The Internet is a vast computer database. As such, its contents must be searched according to the rules of computer database searching. Much 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-born Irish mathematician George Boole.

On Internet search engines, the options for constructing logical relationships among search terms often modify the traditional practice of Boolean searching. This will be covered in the section below, Boolean Searching on the Internet.

Boolean logic consists of three logical operators:

  • OR
  • AND
  • NOT

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

OR logic

Venn diagram for OR

college OR university

Question: I would like information about college.

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 396,482
university 590,791
college OR university 819,214

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

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

Venn diagram for OR

college OR university OR campus

For example:

Search terms Results
college 396,482
university 590,791
college OR university 819,214
college OR university OR campus 929,677

AND logic

Venn diagram for AND

poverty AND crime

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

Here is an example of how AND logic works:

Search terms Results
poverty 76,342
crime 348,252
poverty AND crime 12,998

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

Venn diagram for AND

poverty AND crime AND gender

For example:

Search terms Results
poverty 76,342
crime 348,252
poverty AND crime 12,998
poverty AND crime AND gender 1,220

In addition: a very few search engines make use of the proximity operator NEAR. A proximity operator determines the closeness of terms within the text of a source document. NEAR is a restrictive AND. The closeness of the search terms is determined by the particular search engine. Most search engines default to proximity searching by default.

NOT logic

Venn diagram for NOT

cats NOT dogs

Question: I want information about cats, but I don't want to see anything about dogs.

Here is an example of how NOT logic works:

Search terms Results
cats 86,747
dogs 130,424
cats NOT dogs 65,223

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.

Combined AND and OR logic

Question: I want information about the behavior of cats.

Search: behavior AND (cats OR felines)

You can combine both AND and OR logic in a single search, as shown above.

The 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 process the two related terms as a unit. The search engine will use AND logic to combine this result with the second concept. Using this method, we are assured that the semantically-related OR terms are kept together as a logical unit.

Boolean Searching on the Internet

When you use an Internet search engine, the use of Boolean logic may be manifested in three distinct ways:

  1. Full Boolean logic with the use of the logical operators
  2. Implied Boolean logic with keyword searching
  3. Boolean logic using search form terminology

1. Full Boolean logic with the use of the logical operators

This is classic Boolean searching. However, few search engines nowadays offer the option to do full Boolean searching with the use of the logical operators. It is more common for them to offer simpler methods of constructing search statements, specifically implied Boolean logic and search form terminology. These methods are covered below.

If you want to construct searches using Boolean logical operators, you will need to experiment with search engines and see what happens. You can try some of the search statements shown below. Keep in mind that the search engine might require that the Boolean operators be typed in CAPITAL LETTERS. Don't forget that most search engines provide help pages that explain the kind of searching you can do on their sites.

Question: I need information about cats.

Boolean logic: OR

Search: cats OR felines

Question: I'm interested in dyslexia in adults.

Boolean logic: AND

Search: dyslexia AND adults

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

Boolean logic: NOT

Search: radiation NOT nuclear

Question: I want to learn about cat behavior.

Boolean logic: AND, OR

Search: behavior cats OR felines

The last example is a tricky search on most search engines. It combines implied AND logic - designated by the space between the word "behavior" and "cats" - with the use of the Boolean OR operator. You can give this search a try, but the search engine might not process it as you intended. The safest way to conduct this type of search is to use the advanced search page available on most search engine sites. This will be shown in option #3 below (search form terminology).

2. Implied Boolean logic with keyword searching

keyword searching refers to a search type in which you enter terms representing the concepts you wish to retrieve. Boolean operators are not used.

implied boolean logic refers to a search in which symbols are used to represent Boolean logical operators. In this type of search, the absence of a symbol is also significant, as the space between keywords defaults to either OR logic or AND logic. Nowadays, virtually all general search engines on the Internet default to AND logic.

Implied Boolean logic is so common on Web search engines that it can be considered a de facto standard.

Question: I need information about cats.

Boolean logic: OR

Search: [None]

There are probably no general search engines on the Web that interpret the space between keywords as the Boolean OR. Rather, the space between keywords is interpreted as AND. To do an OR search, choose either option #1 above (full Boolean logic) or option #3 below (search form terminology).

Question: I'm interested in dyslexia in adults.

Boolean logic: AND

Search: dyslexia  adults

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

Boolean logic: NOT

Search: radiation  -nuclear

Notice the minus sign (-) in front of the word "nuclear".

Question: I want to learn about cat behavior.

Boolean logic: AND, OR

Search: [none]

3. Boolean logic using search form terminology

Many search engines offer an advanced search page with a search form which allows you to choose the Boolean operators from a menu. Usually the logical operator is expressed with substitute terminology rather than with the operator itself.

Question: I need information about cats

Boolean logic: OR

Search: Any of the words/At least one of the words/Should contain the words

Question: I'm interested in dyslexia in adults.

Boolean logic: AND

Search: All of these words/Must contain the words

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

Boolean logic: NOT

Search: Must not contain the words/Should not contain the words

Question: I want to learn about cat behavior.

Boolean logic: AND, OR

Search: Combine options if the form allows multiple search logic, as in the example below.

complex advanced search

Quick Comparison Chart:
Full Boolean vs. Implied Boolean vs. Search Form

  Full Boolean Implied Boolean Search Form Terminology
OR college or university [rarely available] any of the words
at least one of the words
should contain the words
AND poverty and crime poverty   crime all of these words
must contain the words
NOT cats not dogs cats   -dogs must not contain the words
should not contain the words
NEAR, etc. cats NEAR dogs N/A near

Where to Search:
A Selected List

Feature Search Engine
Boolean operators Google [OR only] | Ixquick
Full Boolean logic with parentheses, e.g.,
behavior and (cats or felines)
Usually available on the advanced search page
Implied Boolean Most search engines offer this option
Boolean logic
using search form terminology
Most advanced search pages offer this option
Proximity operators Exalead