Structured Query Language/About the Book
It's a Translation and a Guide edit
This Wikibook introduces the programming language SQL as defined by ISO/IEC. The standard is — similar to most standard publications — fairly technical and neither easy to read nor understandable. Therefore there is a demand for a text document explaining the key features of the language. That is what this wikibook strives to do: we want to present an easily readable and understandable introduction for everyone interested in the topic.
Manuals and white papers from database vendors are mainly focused on the technical aspects of their product. As they want to set themselves apart from each other, they tend to emphasize those aspects which go beyond the SQL standard and the products of other vendors. This is contrary to the Wikibook's approach: we want to emphasize the common aspects.
The main audience of this Wikibook are people who want to learn the language, either as beginners or as persons with existing knowledge and some degree of experience looking for a recapitulation.
What this Wikibook is not edit
First of all, the Wikibook is not a reference manual for the syntax of standard SQL or any of its implementations. Reference manuals usually consist of definitions and explanations for those definitions. By contrast, the Wikibook tries to present concepts and basic commands through textual descriptions and examples. Of course, some syntax will be demonstrated. On some pages, there are additional hints about small differences between the standard and particular implementations.
The Wikibook is also not a complete tutorial. First, its focus is the standard and not any concrete implementation. When learning a computer language it is necessary to work with it and experience it personally. Hence, a concrete implementation is needed. And most of them differ more or less from the standard. Second, the Wikibook is far away from reflecting the complete standard, e.g. the central part of the standard consists of about 18 MB text in more than 1,400 pages. But you can use the Wikibook as a companion for learning SQL.
How to proceed edit
For everyone new to SQL, it will be necessary to study the chapters and pages from beginning to end. For persons who have some experience with SQL or are interested in a specific aspect, it is possible to navigate directly to any page.
Knowledge about any other computer language is not necessary, but it will be helpful.
The Wikibook consists of descriptions, definitions, and examples. It should be read with care. Furthermore, it is absolutely necessary to do some experiments with data and data structures personally. Hence, access to a concrete database system, where you can do read-only and read-write tests, is necessary. For those tests, you can use our example database or individually defined tables and data.
Conventions edit
The elements of the language SQL are case-insensitive, e.g., it makes no difference whether you write SELECT ..., Select ..., select ... or any combination of upper and lower case characters like SeLecT. For readability reasons, the Wikibook uses the convention that all language keywords are written in upper case letters and all names of user objects e.g., table and column names, are written in lower case letters.
We will write short SQL commands within one row.
SELECT street FROM address WHERE city = 'Duckburg';
For longer commands spawning multiple lines we use a tabular format.
SELECT street
FROM address
WHERE city IN ('Duckburg', 'Gotham City', 'Hobbs Lane');