Boolean operators OR, AND & NOT
Search terms and search strings are usually combined using Boolean operators OR, AND, and NOT.
This is how the operators work:
Milk OR cream searches for references which include either the term milk or cream or both. The OR operator broadens the search results. It is used, for example, to combine synonyms of words and to combine broader and narrower terms.
Milk AND cream searches for references which include both the search terms milk and cream. AND-operator reduces the search results. It is used, for example, when one wants to combine two different topics in the search.
Milk NOT cream searches for references which include the term milk, but exclude the references with the term cream - references with both terms milk and cream are also excluded. NOT-operator can be used to eliminate a term from appearing in the search results. However, you should be extremely careful in using it as you risk losing good references in the process.
The order of the search of the words within the string depends on the used database. The order can, however, be defined by using parentheses. Parentheses define the order by first combining the search terms inside them. Without parentheses the order is determined by the database.
When, for example, you want references which, in addition to sugar, include either milk or cream.
- The search (milk OR cream) AND sugar will always work correctly.
- Instead, the result of the search milk OR cream AND sugar depends on the database:
Databases also have search fields which define the order between AND and OR operators depending on the database. For example, in the advanced search in OULA database the search fields are combined by always performing AND operator first, and in IEEE Xplore database the order of the search fields moves down from the first one.
You can check the order of the operators in the instructions of the database from the Database guides.