TAILIEUCHUNG - SQL PROGRAMMING STYLE- P34

SQL PROGRAMMING STYLE- P34:Im mot trying to teach you to program in SQL in this book. You might want to read that again. If that is what you wanted, there are better books. This ought to be the second book you buy, not the first. I assume that you already write SQL at some level and want to get better at it. If you want to learn SQL programming tricks, get a copy of my other book, SQL for Smarties (3rd edition, 2005). | 52 CHAPTER 3 DATA DECLARATION LANGUAGE verified for syntax or check digits inside itself. Example The open codes in the UPC scheme that a user can assign to his or her own products. The check digit still works the same way but you have to verify the codes inside your own enterprise. If you have to construct a key yourself it takes time to design it to invent a validation rule and so forth. There is a chapter on that topic in this book. Chapter 5 discusses the design of encoding schemes. 3. An exposed physical locator is not based on attributes in the data model and is exposed to the user. There is no way to predict it or verify it. The system obtains a value through some physical process in the storage hardware that is totally unrelated to the logical data model. Example IDENTITY columns in the T-SQL family other proprietary nonrelational auto-numbering devices and cylinder and track locations on the hard drive used in Oracle. Technically these are not really keys at all because they are attributes of the physical storage and are not even part of the logical data model but they are handy for lazy non-RDBMS programmers who don t want to research or think This is the worst way to program in SQL. 4. A surrogate key is system generated to replace the actual key behind the covers where the user never sees it. It is based on attributes in the table. Example Teradata hashing algorithms pointer chains. The fact that you can never see or use them for DELETE and UPDATE or create them for INSERT is vital. When users can get to them they will screw up the data integrity by getting the real keys and these physical locators out of sync. The system must maintain them. Notice that people get exposed physical locator and surrogate mixed up they are totally different concepts. Auto-Numbers Are Not Relational Keys In an RDBMS the data elements exist at the schema level. You put tables together from attributes with the help of a data dictionary to model entities in SQL. .

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