Because of SQL’s prevalence, you are likely to encounter SQLin multiple products and
environments. One of the (perhaps valid) criticisms of SQLis that while it is similar across
products, there are subtle differences. These differences result from different interpretations of
the standard, different development styles, or different underlying architectures. To understand
these differences, it is helpful to have examples that compare and contrast the subtle differences
in SQLdialects. Effective SQL provides a Rosetta Stone for SQLqueries, showing how queries
can be written in different dialects and explaining the differences.
I often claim that the best way to learn something is by making mistakes. The corollary to this
claim is that the people who know the most have made the most mistakes and have learned from
others’ mistakes. This book includes examples of incomplete and incorrect SQLqueries with
explanations of why they are incomplete and incorrect. This allows you to learn from mistakes
others have made.
SQLis a powerful and complex database language. As a database consultant and a participant in
both the U.S. and international SQLStandards committees, I’ve seen a lot of queries that did not
take advantage of SQL’s capabilities. Application developers who fully learn SQL’s power and
complexities can take full advantage of SQL’s capabilities not only to build applications that
perform well, but also to build those applications efficiently. The 61 specific examples in
Effective SQL assist in this learning.

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