Neither primary nor secondary articles are easy to isolate during database searching. Most databases do not identify primary articles as such.
Review articles may be identified as such or as "historical" articles and can sometimes be isolated in scientific databases by selecting the "review" article (or similar) option on advanced search screens. However, because not all review articles are identified, many important articles can be missed using this method. Doing a keyword search for "review," "systematic review," "review article," etc., is another way to find review articles.