Boolean operators are specific words used to combine keywords to improve the chances of finding relevant information when searching databases. The most commonly used Boolean operators are AND, OR & NOT.
AND is used to join words or phrases when both (or all) the terms must appear in the items you retrieve. The more keywords you combine using AND, the narrower your search results.
OR is used to join synonyms or related terms. Your search results must contain either (or both) of the terms. The more keywords you combine using OR, the broader your results.
NOT is used to exclude a particular word or phrase from your search results. Be cautious using this as it is easy to unintentionally exclude relevant results.
You may find this video on using Boolean operators useful - follow link below: