Retrieve the field with the largest range

in general, the subject field is the largest. The most common fields in the database are Title, Author, Subject, abstract (abstrac), Keywords, Source, etc., and some databases that search for specialized literature types also have some fields unique to this kind of literature, such as patent number of patent literature, standard number of standard literature, tutor of degree thesis, degree-granting school and conference. Some databases don't display fields in the simple retrieval interface. Generally, the default in this case is "all fields" or "full-text retrieval" with the largest range. If you pay attention to fields, you can find that the default retrieval fields of databases marked with fields are mostly "all" or "subject".