Module 2: How to formulate a good search query?

You are here

2.1. Boolean operators (AND, OR, NOT)


Used to search only for information that contains both search terms. When simply typing multiple search terms one after another, the search engine will combine these search terms automatically with the Boolean AND-operator. In mathematics this is called the intersection.


  • Example: estrous cycle AND birds
  • Benefit: many search results are relevant.
  • Disadvantage: not all relevant search results found.


Used to search for information that contains at least one of both search terms. In mathematics this is called the union.


  • Example: estrous cycle OR birds
  • Benefit: many relevant search results found.
  • Disadvantage: many non-relevant search results found as well.


Used to search for information that contains the first but not the second search term. In mathematics this is called the set difference.


  • Example: estrous cycle NOT birds
  • Benefit: fewer and more relevant search results.
  • Disadvantage: risk to exclude relevant search results.


  • Not to be confused with normal spoken language. When you say 'I want to know everything about cats and dogs', you are looking for information on cats or dogs, or both animals. Thus, that 'AND' in spoken language corresponds to the Boolean OR-operator.
  • When combining multiple Boolean operators with each other, use brackets. These give priority to the search terms in brackets and clarify the search query.
    • Note that Google only works with the -sign, not with using ‘NOT’. For example: jaguar speed -car or pandas
    • NOTE: use capital letters for operators, otherwise the search function will recognize them as 'normal' keywords.