TAILIEUCHUNG - SQL PROGRAMMING STYLE- P30

SQL PROGRAMMING STYLE- P30: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). | 72 CHAPTER 4 SCALES AND MEASUREMENTS usually apply to discrete attributes. Nominal scales for continuous attributes can be modeled but are rarely used. Range A scale also has other properties that are of interest to someone building a database. First scales have a range What are the highest and lowest values that can appear on the scale It is possible to have a finite or an infinite limit on either the lower or the upper bound. Overflow and underflow errors are the result of range violations inside the database hardware. Database designers do not have infinite storage so we have to pick a subrange to use in the database when we have no upper or lower bound. For example few computer calendar routines will handle geologic time periods but then few companies have bills that have been outstanding for that long either so we do not mind. Granularity Accuracy and Precision Look at a ruler and a micrometer. They both measure length using the same scale but there is a difference. A micrometer is more precise because it has a finer granularity of units. Granularity is a static property of the scale itself how many notches there are on your ruler. In Europe all industrial drawings are done in millimeters the United States has been using 1 32nd of an inch. Accuracy is how close the measurement comes to the actual value. Precision is a measure of how repeatable a measurement is. Both depend on granularity but they are not the same things. Human nature says that a number impresses according to the square of the number of decimal places. Hence some people will use a computer system to express things to as many decimal places as possible even when it makes no sense. For example civil engineering in the United States uses decimal feet for road design. Nobody can build a road any more precisely than that but many civil engineering students turn in work that is expressed in ten-thousandths of a foot. You don t use a micrometer on asphalt A database often does not give the

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