Gonzaga University: Shawn Bowers' "Relational Algebra and Complex SQL"
Read slides 3 through 33 (pages 2-17). As you read this lecture be sure to think of how the necessary mathematical operations are likely to affect the outcome of your database actions.
Click http://web.archive.org/web/20150616070206/http://www.cs.gonzaga.edu/~bowers/courses/cpsc421-f09/lecture-3.pdf link to open resource.