Most library databases use Boolean operators (AND, OR, and NOT).
Use them to narrow or broaden search results.
Example:
Iran AND China AND (energy OR petroleum OR oil)
Adapted from SAIS Library, Johns Hopkins Univ. "Database Search Tips" Guide
Using AND:
Note: in most, but not all, databases, the AND is implied. For example, Google automatically puts an AND in between search terms.
Example: renewable energy AND China
Using OR:
Example: renewable energy OR solar OR wind
Using NOT:
Example: peacekeeping NOT United Nations
Adapted from SAIS Library, Johns Hopkins Univ. "Database Search Tips" Guide