In this post I like show the origin of Relational database model. There are many books, papers, conferences about this subject, but I find out recently a serie of (very good) videos from CJ Date, one of fathers of the Relational model. Please follow the next link. No problem if you have a lot of years of experience in Relational model (I have about 20 years) and you think you know all about it.
HTH – Antonio NAVARRO