Use the Boolean operators: AND, OR, NOT

The Boolean operators define the relationship between the search words. With the help of these operators we can expand or narrow the list of results. The most frequently used operators are:  AND; OR; NOT

AND combines different terms when both or all must be present.  Search results will contain both or all of the terms. Using AND decreases the number of results, because each time a keyword is added, the focus of the search is narrowed

OR combines synonymous terms when at least one must be present. Search results will contain one or both of the terms. Using OR increases the number of results, because a broader range of keywords is searched.

NOT  eliminates irrelevant tems from search. Search results will contain the first term. Using NOT reduces the number of results by excluding information not required.

In case we are using more than one operator in one search we need to get information about what priority of opeartors in the given database or search engine. Databases show variety in this question, some use AND as the primary operator, in others the OR or NOT operator weighs more in searches.  Related information can be found in the Help menu.

In case the order of operators is predetermined, we can use paranthesis to designate the precedence among them: e.g (humour AND therapy) NOT children

Some databases allow the use of other operators, like XOR; NEAR, NEXT, etc.