Understanding SQL's underlying theory is the best way to guarantee
that your SQL code is correct and your database schema is robust
and maintainable. On the other hand, if you're not well versed in
the theory, you can fall into several traps. In SQL and Relational
Theory, author C.J. Date demonstrates how you can apply relational
theory directly to your use of SQL. With numerous examples and
clear explanations of the reasoning behind them, you'll learn how
to deal with common SQL dilemmas, such as:
*Should database access granted be through views instead of base
tables?
*Nulls in your database are causing you to get wrong answers.
Why? What can you do about it?
*Could you write an SQL query to find employees who have never
been in the same department for more than six months at a time?
*SQL supports "quantified comparisons," but they're better
avoided. Why? How do you avoid them?
*Constraints are crucially important, but most SQL products
don't support them properly. What can you do to resolve this
situation?
Database theory and practice have evolved since Edgar Codd
originally defined the relational model back in 1969. Independent
of any SQL products, SQL and Relational Theory draws on decades of
research to present the most up-to-date treatment of the material
available anywhere. Anyone with a modest to advanced background in
SQL will benefit from the many insights in this book.

http://www.amazon.com/exec/obidos/tg/detail/-/0596523068/

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