As I understand it, polymorphic relationships among tables in a database allow one table to have one foreign key that references many tables.
I have found that tags are a common application of this structure. Many tables may have a tag, and a polymorphic relationship allows each of these tables to reference a single tags table.
Unfortunately, I have not been able to find very much background on this topic, either on google or on this site. Most of the writing is either too general (c.s. theory) or too specific (implementation).
I would like to learn more about their use, and how these structures compare to alternatives. Is there a good reference on the use of polymorphic associations in databases?