tailieunhanh - SQL PROGRAMMING STYLE- P28

SQL PROGRAMMING STYLE- P28: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). | 82 CHAPTER 4 SCALES AND MEASUREMENTS You also need to consider laws and accounting rules that deal with currencies. The European Union has rules for computing with euros and the United States has similar rules for dollars in the Generally Accepted Accounting Practices GAAP . 4. Try to store primary units rather than derived units. This is not always possible because you might not be able to measure anything but the derived unit. Look at your new tire gauge it is set for Pascal Newtons per square meter and will not tell you how many square meters you have on the surface of the tire or the force exerted by the air and you simply cannot figure these things out from the Pascals given. A set of primary units can be arranged in many different ways to construct any possible derived unit desired. Never store both the derived and the primary units in the same table. Not only is this redundant but it opens the door to possible errors when a primary-unit column is changed and the derived units based on it are not updated. Also most computers can recalculate the derived units much faster than they can read a value from a disk drive. 5. Use the same punctuation whenever a unit is displayed. For example do not mix ISO and ANSI date formats or express weight in pounds and kilograms in the same report. Ideally everything should be displayed in the same way in the entire application system. Data Encoding Schemes . . . . . . . ou do not put data directly into a database. You convert it into an encoding scheme first then put the encoding into the rows of the tables. Words have to be written in an alphabet and belong to a language measurements are expressed as numbers. We are so used to seeing words and numbers that we no longer think of them as encoding schemes. We also often fail to distinguish among the possible ways to identify and therefore to encode an entity or property. Do we encode the person receiving medical services or the policy that is paying for them That might depend .