Boolean Operators are three conjunctions used (universally in database search architecture) to combine separate search terms and phrases.
The terms are:
• AND
• OR
• NOT
Each operator combines terms differently from the others and yields dramatically different search results.
AND (aka: selective combiner)
• Principle: AND retrieves only records in which both designated terms are present.
• Retrieval Logic: AND = (A+B), but not A, B (records where one but not both terms are present).
• Example Search: retrieves only records with both "Space" AND "Elevator" present (somewhere in the record).
• Expect: smaller retrieval (as compared with OR, NOT) but high topic relevance of results.
OR (aka: the "All of the Above" option)
• Principle: OR retrieves records where both terms are present, or either term is present.
• Retrieval Logic: OR = (A+B), or A, or B
• Example Search: retrieves records where either "Space" OR "Elevator" are present.
• Expect: large search retrieval with highest percentage of irrelevant records.
NOT (aka: the excluding combiner)
• Principle: NOT eliminates all records containing de-selected term from search retrieval (including records where desired term coincides).
• Retrieval Logic: NOT = A, NOT B, NOT (A+B)
• Example Search: retrieves records for "Space," but not "Elevator," or "Space Elevator."
• Expect: large search retrieval with all records containing deselected terms excluded.
Comparison AND & OR search
The following example search in Academic Search Complete illustrates the difference in search retrieval between the AND & OR operators.
Enter and search the following terms varying the operator thusly:
"wind" AND "farm"
"wind" OR "farm"
Q: which operator retrieved the largest result?
Q: which operator retrieved the smallest result?
The only variable in the two searches above was the choice of boolean operator.
Takeaway:
• The operator with which terms are combined dramatically affects search results.
What if No Operator is Specified in a Search?
Enter and search the terms "wind" & "farm" without specifying a boolean operator between them.
Compare with the foregoing search results.
Results will match the retrieval for the AND search above.
Reason?
• AND is the most commonly used operator in database searching.
• Most databases assume an "AND search" is intended when no operator is specified.
Comparison AND & NOT search
Still in Academic Search Complete, enter and search the following:
"wind" AND "farm"¹
"wind" NOT "farm"
Note: record tally for AND search.
Delete "farm" from strategy and search "wind" alone.
Add together record tallys for AND & NOT searches above & compare with single term "wind" record tally.
Q: Is there any numerical difference between the numbers?
The numbers should match perfectly.
Reason?
• The single-term "wind" search yields the total number of records in the database containing the word "wind" somewhere in their record.
• Combining the record tallys for the AND & NOT searches adds up to this total number because...
1. the NOT result indicates the total number of records, with the term "farm" removed
2. the AND result indicates the total number of records in which the terms "wind" and "farm" appear together.
Takeaway: NOT retreives all records for a search term, except those containing any instance of an excluded term.
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¹ If still in the same Academic Search Complete session, click the Search History link to access your temporarily stored search result for "wind AND farm."