SQL for Web Nerds
by Philip Greenspun
This is a free online SQL book by Philip Greenspun. It covers data modeling, simple queries, complex queries, transactions, triggers, views, style, PL/SQL and Java, trees, handling dates in Oracle, limits in Oracle, tuning, data warehousing, foreign and legacy data and normalization.
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Download or read it online for free here:
by Rahul Batra - dreamincode.net
This is a short e-book teaching the basics of SQL assuming no prior knowledge of it. While the examples and text use Ingres as the database management system, the code should work correctly on any SQL compliant database.
by Mark McIlroy - Blue Sky Technology
An introduction to the use of SQL to retrieve data and generate query result tables. The book covers SQL queries and is suitable for beginners and intermediate users. This includes an on-line environment where you can try out the examples.
by Akeel I. Din - NCC Blackwell
Introductory text for learning SQL, it shows you how to use the language with crystal-clear explanations and examples. For more advanced and vendor-specific topics (triggers, stored procs, etc.) you'll need to look at the vendor's documentation.
by Markus Winand
SQL performance problems are everywhere, everyday. This book is a free SQL tuning e-book for developers. It avoids unnecessary details about database internals but highlights the one topic that is most often neglected: proper indexing.