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PART III
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ORDER_FACTS day_key customer_key product_key salesrep_key . . . order_dollars . . . CUSTOMER customer_key customer_id customer_name industry_group_key . . . INDUSTRY_GROUP industry_group_key
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INDUSTRY industry_key industry_code industry_name . . .
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GROUP_MEMBERSHIP This table has only one column! industry_group_key industry_key primary_industry
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Figure 9-11 Resolving the many-to-many relationship
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TIP If you have a software product that will not allow a many-to-many relationship, you can create an intersect table to resolve it. It looks silly, but it works. The industry_group table in this example is entirely redundant. All information is present in the group_membership table. If you were to look at its content, it would be nothing more than a sequence of integers. Nevertheless, this design will satisfy tools that require a primary key foreign key relationship to drive each join. Another way to resolve the many-to-many relationship is to remove the group key from the dimension table and replace the bridge with a simple intersect table. Each row in the intersect table contains foreign keys that refer to a row in the dimension and a row in the outrigger. With industry groups, for example, the customer table and industry table are linked by an intersect table that contains a customer_key and industry_key. No group key is present in the customer table. The intersect table in this solution may grow larger than the one in Figure 9-11, since groupings cannot be not reused by more than one customer.
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Bridge as Fact Table Because it consists primarily of foreign keys, a bridge table bears a strong resemblance to a fact table. Although it contains no facts, it serves the same purpose: capturing the relationship between attributes in the related tables. In Figure 9-10, industry_ group resembles a fact table; it contains multiple key references to other tables. This resemblance is even stronger for the group_membership table in Figure 9-11, where the many-to-many relationship is resolved.
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NOTE There is such a thing as a factless fact table, and the concept is fully explored in 12. Many dimensionally aware software products will identify bridge tables as fact tables. If they contain allocation factors or flags, these may be identified as degenerate dimensions. It is possible to argue the pros and cons of referring to a bridge table as a fact table, but it
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9 Multi-Valued Dimensions and Bridges 217
would be an exercise in semantics only. Regardless of what you call it, the bridge table serves its purpose. There is, however, one significant consideration. If you choose to call the bridge a fact table, or if your toolset makes that choice for you, you will need to relax a restriction that was introduced in 4. As you may recall, that chapter explored the consequences of joining two fact tables together. Because this kind of query can result in a Cartesian product, or double-counting, 4 cautioned not to do it, and instead recommended following drill-across procedures. As you have seen in this chapter, a bridge table exploits this same Cartesian effect in a useful and powerful way. To call it a fact table, then, will require making an exception to the rule not to join fact tables. When one of the fact tables is a bridge, the join is essential and the Cartesian product is intended.
Summary
This chapter has provided tools to address real-life complexity that cannot fit within the traditional one-to-many relationship between dimension table and fact table. A quick recap of the primary lessons: When a fact must refer to more than one instance of a dimension, it may be possible to simplify the many-to-many relationship into multiple one-to-many relationships. This simplification may be less than satisfying, because it makes several types of reporting more difficult and only supports a fixed number of dimension values. A bridge table allows a fact table row to be associated with any number of dimension rows, and keeps the reporting simple. The bridge table makes it very easy to double count facts, so access and usage must be carefully controlled. A bridge may contain an allocation factor or be supplemented by a simplified relationship that isolates a single instance in a primary role. The many-to-many relationship introduced by the bridge can be resolved via an intersect table if required by your toolset. The presence of a bridge makes it necessary to think carefully about the implications of slow changes, not only to attributes but also to relationships. An attribute bridge works the same way as a dimension bridge but is placed between a dimension table and an outrigger. Some tools identify bridge tables as fact tables because they carry multiple foreign keys. This is fine, but be sure that the tool does not decide to drill across. Slow change considerations impact bridged solutions in interesting ways. A type 2 change can have a ripple effect that will impact multiple rows in multiple tables. This is not the end of the story on the powerful but dangerous bridge table. The next chapter will show you how a hierarchy bridge can be used to represent a recursive relationship among dimension members, allowing facts to be rolled up or down to any level in a ragged hierarchy.
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