Publishers have started to assign a Digital Object Identifier (DOI) to journal articles, electronic books and other types of documents. A DOI is a unique alpha-numeric code given to individual online works. The DOI usually appears at the top of the article or in the database record for the article.
At the moment, not all publishers provide DOIs. Some library databases such as PsycINFO provide a DOI for each article. In Chicago Style, when a DOI is provided, your should provide it at the end of your reference. Items with a DOI do not include the database name in the citation.
For articles from a library database, without a DOI, "list the name of the commercial database rather than the URL." (Chicago Manual of Style, 17th ed., p. 833).