[pjungwir]

I just finished Database in Depth. :-) It is short and readable. It's targeted to working database programmers with a few years of SQL already. I learned a bit, but it was a bit of a bait-and-switch: the book is really a polemic about why SQL is not relational enough and people should use the author's own Tutorial D. Every page belabors that point. More examples are written in Tutorial D than SQL. I was hoping for some math & theory, and there was some theory, but there was little effort to connect it to SQL except to show SQL's shortcomings. For instance I never once saw the term "outer join" (because Date wants to forbid NULLs). If you are interested in database theory and trying new things, it might be very interesting. If you are a working programmer who wants to solidify the foundations, there are probably better choices (I hope).

[dragonwriter]

On the kernel language topic, though, Date and Darwen's work [0] on the specifications for an ideal class of database languages unifying OO and Relational models (D; of which Tutorial D is one realization intended principally for pedagogical purposes, hence the name) and their lower level work on an abstract relational algebra (A) to underlie D is probably useful in that regard, as frustrating as some of their work may be to people who just want to get down to using current SQL-based relational DBs.

[0] available at http://www.thethirdmanifesto.com/