A complete SQL reference for everyone using one or more of four popular relational databases.
Obviously I didn't sit down and read this book from cover to cover as doing so would have been extremely boring. The book isn't designed for that purpose because its sole purpose is to provide a reference to the SQL language - something that this book does extremely well. It is just unfortunate that the author decided to drop two implementations from the coverage in this edition (which makes the book smaller without making it less useful if you are using one or more of the four implementations it still covers in full detail but which makes the book a lot less useful than the previous edition if you are using one of the two implementations that were dropped).
For those only interested in one implementation of relational databases that use SQL there is a lot of surplus information about three implementations that you are not using but this info can still be helpful in determining how portable your SQL is to cater form moving to a different implementation should that become necessary in the future.
The amount of material on SQL that the book covers means that some compromise was needed and rather than compromising on the coverage of SQL itself the author decided to compromise on the number of implementations that are detailed. This doesn't make the book useless to those using databases other than the four covered, it just means that not everything will work exactly the way the book says. Most of the actual SQL information will be applicable to any SQL database and the four implementations that are covered will give clues to what might be different. A reasonably good compromise for everyone except those who were using the implementations that have been dropped from this edition.