You will need to be consistent in searching the different databases, by using the same keywords/terms.
For example, don't search for (mice AND cancer AND liver) in database 1, then search for (mice AND tumor AND liver) in database two.
However, the way you structure or format the search string will probably have to vary in different databases. Most databases use Boolean searching techniques, but they differ in which symbols you need to use for certain operations. Below is a general guide on how the searching works, so you can be precise in what you look for, but please consult the database's help page or ask a librarian on the specifics.
Boolean operators allow you to combine terms using AND, OR, and NOT. Details and examples for each operator are below.
The majority of databases will use Boolean searching in some way.