A search can be narrowed down by focusing it on the different fields of the record, e.g. title, abstract, keywords, authors or publication year. Depending on the discipline, databases may contain, for example, formulae of chemical compounds, DNA sequences or images. In addition, in some databases search results are grouped according to document types, so it is possible to focus, for example, on only scholarly journals. The contents of the fields must always be checked from the instructions of the database, because the fields have different names depending on the databases and user interfaces. Databases may have field specific alphabetical indices of the words fed into the fields. Notice, that not all documents contain all fields.
Normally, databases offer search forms in which fields can be selected from drop-down menus, or search boxes are offered in which command searches and codes for field restrictions can be used. There are often more search fields available in command search.
When focused on all search fields, a free text search can produce a large search result containing also irrelevant references. When searching for information on a particular topic, in general it is advisable to limit the search to the most important fields from the point of view of the subject matter, i.e. the title, abstract and descriptor fields. It is worth using a full text search when looking for factual information, or when the topic is well defined, or it is otherwise difficult to find information on.
A Free Text Search focused on all fields using the term "decision support system*" produces references where the search terms can appear in any field: in the example reference, the term can be found in the document title, the Publication title, the Abstract and the Subjects.
Image source: ProQuest <http://search.proquest.com> 5.7.2013
On the Advanced Search search form in the ProQuest interface, the field code can be selected from a drop-down menu. The field Anywhere except full text focuses the search simultaneously on all fields and field Anywhere also on full texts in full text providing databases. Image source: ProQuest <http://search.proquest.com> 25.7.2013
Databases often use an established subject index or thesaurus. When a document, for example an article, is added to a database, its most essential contents are described with terms chosen from the thesaurus and these terms are entered in the subject term field (Descriptors, Controlled terms, Index terms..) of the reference. Database interfaces normally offer an option for a Subject Heading Search which allows you to limit the search to subject (descriptor) fields. The purpose of creating descriptors is to help those in need of information to find relevant references.
A Subject Heading Search looks for the terms decision support systems only in the Subjects field.
Image source: ProQuest <http://search.proquest.com> 26.7.2013.
In many databases, classification systems are also used for describing the contents of documents. The classification codes can be seen in each record in its own field (classification codes, cc, ....) and these can also be made use of in information retrieval when focusing the search.
MathSciNet database uses MSC Classification (Mathematics Subject Classification). A classification search provides you with references, where in the 'classification codes' field we retrieve results containing the code 60.65. Image source: MathSciNet<http://www.ams.org/mathscinet/> 2.7.2013
In addition to codes selection Scopus Advanced search for example has SUBJTERMS(2216) field, where 2216 is ASJC-code (All Science Journal Classification) limiting the search into specific seubject area, in this case architecture. ASJC-codes can be found in Scopus Titlelist -exel on page http://www.info.sciverse.com/scopus/scopus-in-detail/facts.
Image source: Scopus <http://www.scopus.com> 22.8.2014