A DOI, or Digital Object Identifier, is a standardized string of letters and numbers that represent an online resource. In APA style, articles or other materials found in a database are not generally cited with the name of the database in which it was found. When possible, a DOI, is given. In practice DOIs are mainly available for scholarly journal articles, and not always then.
As mentioned above, the DOI is available mainly for scholarly journals. That means that a database that includes magazines or reference books as well as scholarly (peer-reviewed) journals may have some articles with DOI and some without.
The DOI is sometimes given as part of the text of an article, above or below the abstract. See also under PDF files, below.
The DOI should be given in the citation in the approved URL form. Some articles may have the DOI listed with the prefix DOI and without the doi.org domain:
Remove the DOI: and substitute https://doi.org/
Click on the title of the article to display the full text. The DOI is shown after the authors' names.
Some journals print a DOI for each article, usually at the top or bottom of the first page of the article, or near the abstract.
If there is a periodical article without a DOI, the APA style requires you to omit it if the article is from a library database. If it is a web page outside of a library database, give the URL of the article.