Leer libro Título: UNDERSTANDING SQL
Año: 1990
Formato: PDF

This book is intended to be an introduction to the SQL database manipulation language. The coverage is not meant to be exhaustive; however, after reading the book you will have a good overall view of the concepts behind SQL and a working knowledge of its commands.

Although the book is based around the 1986 ANSI standard SQL, you will soon find out that no current SQL product implements the standard faithfully. We have tried to show you the ANSI commands, and in addition point out areas where some of the major SQL implementations differ (often for very good reasons) from the standard, and indeed, from each other.

We chose to use DB2, dBASE IV SQL, Informix and Professional Oracle as representatives of the wide range of SQL implementations. DB2 is available on mainframes only; Informix and Oracle on mainframes, minis and Personal Computers (PCs), and dBASE IV SQL only on PCs. Throughout the text you will find ANSI SQL and these four SQLs used and contrasted in examples. Appendix B gives an overview of each product. Chapter 12 gives a chart comparing their main features together with a brief guide to their divergences from the standard, any extra facilities they provide, and a general feel for what they are like to use.

However, this book is not intended to be a comprehensive guide to Oracle, dBASE, Informix and DB2. If you want to know more details about any particular SQL implementation, refer to its manuals, or to a book specifically about that SQL.

Because SQL now comes 'packaged' with dBASE IV, many dBASE IV users will find themselves with a full-featured SQL system 'thrown in' with dBASE. In most cases, those users will have bought dBASE for the dBASE facilities, and not for the SQL features. If you are one of these people, you may well be wondering what SQL can do that dBASE cannot. Your situation is rather different from that of most other users of SQL databases, who have presumably chosen an SQL system for themselves on its own merits. We hope that this book will provide you with a introduction to SQL, and help you see how you might integrate SQL with dBASE IV.

If you have already been exposed to a non-SQL-based system, (for example, if you have previously used PC database systems such as dBASE or Paradox), you will naturally find yourself constantly comparing SQL to these, quite possibly initially to SQL's detriment. But you must realise that such comparisons are like comparing oranges to apples; both have their individual flavours and advantages, but are completely different. SQL and dBASE are alike only in that they both deal with data in a database system; as you will see, SQL is not a competitor to 'traditional' PC-based database systems, but will be used to complement them.

Leer libro