how to make barcode in 2010 Designing the Dimensional Model in Software

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Designing the Dimensional Model
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Although this overview describes design as a discrete step, design work usually begins as soon as the first interview is completed. As the interview process proceeds, the design team will begin to develop an understanding of the key business processes and dimensions that fall within the project scope. Design ideas will begin to gel and evolve. Once all interviews are completed, informal back-of-the-napkin sketches and impromptu white board scribbling must be consolidated into a formal design for review by a larger group. Whether sketching initial ideas or formally recording a proposed dimensional model for a design review, the design team should capture several key elements. After a proposed dimensional design is formulated, it is essential to conduct a design review. This critical project step allows interested parties who are not part of the core design team to provide their own input and participate in key design decisions. It is a grave mistake to accept the draft design of a dimensional modeling expert without such a review.
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18 How to Design and Document a Dimensional Model 441
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Developing the Dimensional Model
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Experienced dimensional modelers tend to think in terms of facts and dimensions, processes and grain, additivity, and conformance. For these people, the development of a draft design is a natural and intuitive process. For the rest of us, it can be useful to follow some basic steps.
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Sketching a Star Kimball and Ross recommend a simple four-step method for sketching out a dimensional design. These steps are a useful way to get started, regardless of whether you subscribe to their recommended architecture. Following these steps takes care of some of the basics. Later in this chapter, you will learn exactly how to document them. As you learned in 4, each star usually corresponds to a discrete process or activity. Review your interview notes, and try to identify individual processes. Pick one, and sketch out a star by following these steps:
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1. Describe the process. 2. Set the grain of the fact table. 3. List the facts. 4. Identify the dimensions. These four steps will get you started, and also force you to consider some essential characteristics of every design. Suppose, for example, that you are doing design work for a sales data mart. Through the course of interviews, you have spoken with a variety of individuals. Reviewing your notes, you are able to list a series of subprocesses that fall under the umbrella of sales: Making sales calls Issuing proposals to customers Booking orders Shipping products Processing returns Paying commissions to salespeople Each of these processes is a candidate for a fact table. Some may or may not be required, and some may turn out to break down into more than one subprocess, but this is a good place to start. Taking one of these processes, you can apply Kimball and Ross s four steps. The specific things you should document are described in more detail later in this chapter. For now, think about each step and what kinds of things to look for. The first step is to name the process. Do this in business terms, if possible: Recording orders from customers. The second step is to establish the grain. As you learned in 3, grain is usually set at the lowest level of detail possible. For orders, you might state it as Order Information at the individual order_line level of detail. As part of this declaration, note the type of fact table implied. In this case, it is a transaction fact table. As you learned in 11, the other kinds of fact tables are snapshots and accumulating snapshots.
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Part VI
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Next, you can enumerate some facts. Remember that you should look for additive facts and not be afraid if some are redundant. Try to define each one and note its additivity characteristics. For orders, you might come up with several facts: quantity ordered, the dollar amount of the order line, and the extended cost associated with the order line (not the unit cost!). The design should also include margin dollars, even though it can be computed as order dollars minus extended cost. Don t be disheartened if you have trouble finding facts. As you learned in 12, some fact tables do not contain any. You will usually be clued into this possibility when you hear people saying they want to count things. Number of applications approved, number of phone calls made, number of sales calls these counts may not manifest themselves as explicit facts, but are implied by factless fact tables with appropriate grain. Note them as part of your design. At this point, you should also be thinking about nonadditive facts that can be computed based on the design. As you will see later in this chapter, it is useful to write them down. If you have already sketched out a design for another star, you may also see some opportunities to drill across, or compare processes. Close rate, for example, might be the ratio of facts in two fact tables: proposals and orders. Write these kinds of facts down, too, because they will be useful. Semi-additive facts are a common feature of snapshot designs, as you learned in 11. If you find any of these, be sure to note the dimension across which they should not be added. The fourth step is to enumerate the dimensions. Do not limit yourself to those that guarantee uniqueness of a fact table row. An order line, for example, is enough to define a fact table row. It may make a fine degenerate dimension, but what other dimensions are implied There is almost always one occurrence of a time dimension, which captures when the activity took place. In this case, it is the order date. Time is sometimes available in other contexts as well. If orders result from proposals, then each order may also have an associated proposal date. Other key dimensions will have revealed themselves as part of the interview process. In this example, they include customer, salesrep, and product. Candidate dimensions will shift and change as you work through and revise a dimensional model. As you learned in 5, dimensions play a central role in ensuring that multiple stars can work together. Moving on to other stars, you will find other processes that share some characteristics with dimensions you have already sketched out. Be on the lookout for dimensions that offer more or less detail than ones you have already laid out. These are likely candidates for conformance. You may also have trouble deciding what elements to include in a single dimension table and what should be split into multiple tables. Review the guidelines in 6 for some help with this.
Beyond Individual Stars The four steps focus on individual stars. After sketching out some stars, step back and look across processes. Three additional steps can guide your next efforts:
1. Work out a conformance framework. 2. Identify the slow change characteristics of dimension tables. 3. Consider alternative models for each process.
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