It is from relational algebra used in database theory. There is an excerpt from one of the first MOOCS offered here on Lagunitas now.[1] It is pretty intuitive

It is from relational algebra used in database theory. There is an excerpt from one of the first MOOCS offered here on Lagunitas now.[1] It is pretty intuitive once you get the hang of it. [1] https://lagunita.stanford.edu/courses/DB/RA/SelfPaced/course/ https://lagunita.stanford.edu/courses/DB/RA/SelfPaced/course...