Database Management 2ed. McFadden FR, Hoffer JA.
(Benjamin Cummings, 1988; ISBN 0-8053-6783-7).
A good introductory text. Now elderly, but still good in parts!
SQL Unleashed. Ladányi, H.
(Sams Publishing, 1997. ISBN 0 672 31133 X)
An intermittently very insightful introduction to SQL, stuffed with goodies,
somewhat skewed towards Oracle, and with a few peculiarities that render
the book less than perfect.
SQL-99 Complete, Really. Gulutzan P & Pelzer T.
As the title suggests - the definitive reference, in our opinion!