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)