Library databases, and other search engines, are often very literal. When you type words into a search box in a library database, the database searches for those exact words. To make your search more effective, pick the major concepts or most important words from your research topic/question, rather than a complete sentence.
Once you have your major concepts selected, think of synonyms or related terms for these major concepts. Google, Wikipedia, dictionaries and thesauri can help with this.
For example, let's say you're researching the use of precision farming by soybean producers in the United States. Some possible major concepts include:
For each of these major concepts or keywords, I can try to think of synonyms or related terms to use in my search. Authors don't always use the exact words you might have in your rearch question/topic. Try to think of words that others might have used, and then combine them using Boolean opeartors (AND, OR and NOT).
Use AND to narrow your search.
When using AND, it will find records that have both terms in them. For example the following will search for articles that have both precision farming and soybeans somewhere in the text when doing a keyword search.
A wildcard tells the database that you will accept other forms of a word. It will expand your search results. To use a wildcard, cut off the end of a word and replace it with an asterisk (*).
agri* → gets you agriculture, agricultural, agribusiness, agriculturist, etc.
biotech* → gets you biotech, biotechnology, biotechnological, etc.
Use OR to expand your search.
When using OR in a keyword search, it will find records that have either or both terms in them. For example the following will search for articles that have either high genetic engineering or genetically modified foods or both.
Sometimes searching just for particular words isn't specific enough and will return many unrelated results. If you only are interested in particular words found in a certain order, you can put quotation marks around them to search them as a phrase. If you are searching in PubMed, Medline or CINAHL, though, it is better to not use quotation marks. They have a feature that aids in searching and is turned off when you use quotation marks.
I'm interested in finding more about precision farming. "Precision farming" will return articles that have those two words next to each other. If I just put in precision farming without the quotes, it will return any article that has precision or farming in it.