If you search a catalog or database and receive a large number of results, add a limit or additional keyword in order to retrieve a manageable and relevant number of results to review. At the same time overly narrow search terms can return too few results. One way of solving both problems is to use Boolean operators (AND, OR, NOT), which allow you to limit or expand searches depending on your needs.
For example, a search for oil AND politics will return items that contain both concepts":
petroleum OR oil returns items that contain either one of the terms or both:
central america NOT costa rica returns items that talk about the region but do not mention the country Costa Rica:
Phrase searching:
An important strategy to use when searching for phrases ("petroleum industry") or titles:
For example, "insurgency in Nigeria"
will search for those words in that order, finding the 2015 book: Oil Wealth and Insurgency in Nigeria by Omolade Adunbi 
Truncation and Wildcards:
Most catalogs and databases enable users to search variations of keywords by using truncation (*) or wildcard (e.g., ?, $, !) symbols.
For example, one could search for politic* to find poltic, politics, political, politicking, and so on.
Wildcard searches are for differences within words: a search for wom?n will return results for woman, women, and womyn.
Nested Searching:
When pairing two or more search words with other concepts, it is important to "nest" the related terms together with parentheses.
For example, (petroleum OR oil) [Subject] AND (politic* OR war OR conflict*) [Subject] Publications 2010-present= 243 results at HC
The search returns results for the union of the two subject areas.