Boolean operators

Boolean operators are used to combine search terms and instruct the database or search engine how to search and retrieve the information you require.

The Boolean operators are AND, OR and NOT. These search conventions are used by most search engines and search tools. You should become familiar with using them as they could make a big difference to the relevance and number of results you get.

cats dogs


AND retrieves only the records containing all of the combined terms: this example has retrieved records about both cats and dogs.
cats dogsset-full


OR retrieves records containing any of the combined terms: this example has retrieved records about either cats or dogs, or both. Records are not duplicated in the results.
cats dogsset-left


NOT retrieves records containing one term but excludes records containing an unwanted term: this example has retrieved records about cats, but has left out anything about dogs. Use NOT with caution, as it can exclude records which may be useful.
Kidney diseaseset-triplecats dogs




Use more than one Boolean operator to make more complex refinements: this example has retrieved records about kidney disease and either cats or dogs.

Some search engines require that you use brackets or inverted commas around the keywords you are combining using the Boolean operator OR. This is referred to as nesting. If you are unsure whether to use them, check the help tab on the resource you are using for searching guides.

For example, you may need to structure your search as:

(cat OR cats OR dog OR dogs) AND kidney disease