Here is a series of videos by CJ Date on Relational Theory.
Introduction video
An Introduction to set theory (videos on O’Reilly website)
Nullology (Videos on O’Reilly website)
Nulls, Three-Valued Logic, and Missing Information (videos on O’Reilly website)
The Closed World Assumption (videos on O’Reilly website)
View Updating (videos on O’Reilly website)
