After you have defined your topic and developed your search terms, the next step is to combine keywords together to create a search query. Remember that when you are searching online, you are making a command to a database.
Boolean searching is a system of logical operations used to search computer databases. It is named after George Boole, a nineteenth century mathematician. There are three commands (aka operators) recognized by most search engines: AND, OR, AND NOT. Think of a Venn diagram.
The AND command returns documents that contain both words used in the search.
Example: dropouts AND teenagers
The OR command returns documents that contain either the first word or the second word. It expands the search and is usually used to find all of the documents on a subject that can be called two or more different keywords.
Example: teenagers OR adolescents
The AND NOT command excludes documents that contain terms. It is usually used when a word can have two meanings and you want to screen out documents based on a particular meaning. For example, the word "java" can refer to both a computer programming language and a type of coffee.
Example: java AND NOT coffee
To chain together a set of commands, parentheses are used to clarify the order of operations, just like a mathematical equation. Alternately, the advanced search screen of many databases will help you group your search terms together appropriately without having to enter in parentheses on your own.
Example: (teenagers OR adolescents) AND dropouts
If you are still having trouble understanding Boolean commands, try this quick tutorial from the Colorado State University Libraries.
To signify that a set of words must be joined together as a set, use the double parentheses command.
Otherwise, many databases will read a search for exact phrase as a search for the terms exact AND phrase appearing anywhere in the document, i.e. the word exact might appear in the first paragraph and the word phrase could appear in the last paragraph.