Guide
Boolean search
Boolean search uses the operators AND, OR and NOT, together with phrase searching, truncation and nesting, to combine search terms precisely and control what a database returns.
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The three Boolean operators
Boolean searching, named after the logician George Boole, rests on three operators. AND retrieves only records that contain every linked term, so it narrows a search and is used to combine different concepts: "adolescents AND depression" finds records mentioning both. OR retrieves records that contain any of the linked terms, so it broadens a search and is used to gather synonyms for one concept: "teenagers OR adolescents". NOT excludes records containing a term: "depression NOT bipolar". NOT is powerful but risky, because it can discard relevant records that happen to mention the excluded word, so most searchers use it sparingly.
Phrase searching, truncation and wildcards
Three more tools refine the operators. Phrase searching, usually with quotation marks, forces the database to treat words as an exact phrase in order: "randomised controlled trial" rather than the three words scattered apart. Truncation, commonly the asterisk, retrieves a stem plus any ending: nurs* finds nurse, nurses and nursing in one term, saving you from listing every variant with OR. Wildcards substitute for a single character to handle spelling variants — for example a symbol within wom?n to catch both woman and women, or to cover British and American spellings. The exact symbols differ between databases, so check each platform’s help.
Nesting with brackets
When a search mixes operators, the order in which they are applied changes the result, so brackets are used to group terms — a technique called nesting. The convention is to put synonyms joined by OR inside brackets, then combine the bracketed groups with AND. For example, ("teenagers" OR "adolescents") AND ("depression" OR "low mood") tells the database to find any of the first group together with any of the second. Without the brackets, the database applies its default operator order and may return something quite different. Nesting is what lets a multi-concept search built from many synonyms behave predictably.
A worked example
Suppose the question is whether exercise reduces anxiety in older adults. There are three concepts: the population, the intervention and the outcome. You gather synonyms for each and combine them: ("older adults" OR "elderly" OR "geriatric") AND (exercise OR "physical activity" OR training) AND (anxiety OR "anxious" OR "panic"). Inside each bracket, OR widens the net for that concept; the AND between brackets requires all three concepts to appear; truncation such as anx* could replace the anxiety synonyms; and quotation marks keep multi-word phrases intact. Documenting this string, database by database, makes the search transparent and reproducible.
Key facts
At a glance
- AND: narrows — records must contain all terms (combine concepts)
- OR: broadens — records contain any term (gather synonyms)
- NOT: excludes a term — use sparingly to avoid losing relevant records
- Phrase: quotation marks force an exact word order
- Truncation: a symbol (often *) retrieves a stem plus any ending
- Nesting: brackets group terms so operators apply in the intended order
Common questions
FAQ
Does AND make a search bigger or smaller?+
AND makes a search smaller. It returns only records that contain every term joined by it, so each additional AND term narrows the results. It is used to combine different concepts — for example a population and an intervention — so that retrieved records are relevant to all of them. To broaden a search instead, use OR to add synonyms.
What does the asterisk do in a search?+
The asterisk is the most common truncation symbol. Placed at the end of a word stem, it retrieves that stem with any ending: child* finds child, children and childhood in a single term. This saves listing every variant with OR and improves a search’s sensitivity. The exact symbol varies by database, so check the platform’s help if the asterisk does not work.
Why do I need brackets in a Boolean search?+
Brackets, or nesting, control the order in which operators are applied when a search mixes AND and OR. By grouping synonyms joined with OR inside brackets and then combining the groups with AND, you ensure the database interprets the logic as intended. Without brackets, the default operator order can produce very different and often wrong results.
