Once you have identified the keywords and major concepts from your research statement that you want to find, you can build a database query using Boolean logic, which uses the combining operators AND, OR, NOT between search terms to sort them for the most effective results.
For example, consider the following search statements using the same variables with AND, OR, NOT:
Advanced Boolean searching: You can use AND, OR, NOT in combination. As a rule of thumb, when you want to search two related terms together in a longer Boolean search statement, connect the with OR and group them in a set of parentheses.
Additionally you can put multi-word terms in quotation marks to make doubly sure the words are searched together as a phrase - but this is optional:
("differential diagnosis" OR psychodiagnosis) AND "major depression" AND adults NOT children
This video from the University of Auckland Libraries in Australia explains Boolean operators very well, and shows how they work in library catalogs, databases, and even Advanced Google.