Searching computer databases follows certain rules most often 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

OR

college OR university

• In this search, we will retrieve records in which AT LEAST ONE of the search terms is present. We are searching on the terms college and also university since documents containing either of these words might be relevant.
• 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 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.

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"

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

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. As another example, Google defaults to proximity searching by default.

NOT

cats NOT 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

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.