By using quotation marks (phrase search) you can search with the combination of two or more words. The words written inside the quotation marks will appear in that order in the search results. There are differences in phrase searches between databases, which you can check in the database quick guide.
When searching without quotation marks, the words are searched individually in the database. Therefore, the search words may occur far away from each other, so the search results may include unusable material.
Boolean operators OR, AND and NOT are usually used to combine search terms and strings. In many databases you can also use proximity operators. Search terms with their truncation symbols and the operators that combine them form a search string, and this is expressed differently depending on the database. In addition to the used operators, combining search terms and strings is affected by the search order and the use of brackets. It is wise to check the operators and their functions in the guides.
With the help of a search string you will get the most versatile search possibilities when you are looking for comprehensive information on a certain topic.
As an example, a search about the topic "environmental effects of plastic waste".
An example search string:
"plastic waste" OR "litter plastic"
environment* OR pollut*
The example picture information has been searched on the environmental effects of plastic waste in Scopus database.
Picture: Scopus <https://www.scopus.com/search> 13.6.2018
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.