Their creators do not specify how the database was created and from what sources. Use these sources with care. If you don't find what you want it doesn't mean the information doesn't exist, perhaps only that the database developer has not added the source to file. You should also know that many on-line databases overlap, but you'll often never find out how much until you start to use them. This is particularly true of databases that contain abstracts from journals.
Many different services exist and they sometimes summarize the same journals.
Boolean Searching in the Internet , from the State University of New York-Albany Study the syntax used by different search engines Search engines differ in the way you write and execute queries. Some provide a form that you fill out. Others require that you type a search string. Before using a search engine or database, study the documentation about the syntax it employs. Quick Reference Guide to Search Engine Syntax, from State University of New York-Albany.