barcode vb.net 2010 Simplifying the Relationship in Software

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Simplifying the Relationship
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The simplest way to deal with a multi-valued dimension is to decompose the many-to-many relationship into two or more one-to-many relationships. This solution makes use of roles, as covered in 6. The fact table will have not one relationship to the dimension table but two or more. For a given fact, each of the relationships will identify exactly one row in the dimension table. In the case of collaboration among salespeople, order_facts can be designed with two salesrep_keys. One is designated as primary, the other as secondary. The resulting design is depicted in Figure 9-3. The figure depicts two aliases for the single salesrep table. Each is connected using the appropriate foreign key in the fact table. If only one salesperson is involved in a particular order, the secondary_salesrep_key will refer to a special row in the salesrep table for not applicable, a technique also described in 6. This design appears to solve the problem, but issues are lurking beneath the surface. First, there are certain forms of reporting that will now be much more complicated. Want to total up order_dollars for a particular salesperson You ll have to filter your query twice, once for the primary salesperson and once for secondary, and link these filters with or. Want to list all salespeople along with the total order_dollars in which they were involved You ll probably need to query the fact table twice, once grouping order_dollars by the primary salesperson, and again grouping order_dollars by the secondary salesperson. Then, you can merge the result sets together. All this becomes more complicated if the fact table allows for three salespeople, or four, or more.
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9 Multi-Valued Dimensions and Bridges 199
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ORDER_FACTS day_key product_key customer_key primary_salesrep_key secondary_salesrep_key
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PRIMARY_ SALESREP salesrep_key salesperson_name
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Alias for: SALESREP Role: Primary Salesperson
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order_dollars CUSTOMER
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SECONDARY_ SALESREP salesrep_key salesperson_name
. . .
Alias for: SALESREP Role: Secondary Salesperson
The second problem is that the solution only allows for a fixed number of relationships. If there are two salesrep_keys in the fact table, for example, then nothing can be done when an order comes along with three salespeople. You might anticipate this and include enough keys for the highest number of collaborators ever encountered, but eventually a new record will be set. TIP A multi-valued relationship between fact and dimension table can be resolved by breaking it down into two or more one-to-many relationships. However, this solution complicates reporting and supports only a finite number of relationships. Although imperfect, this solution may be acceptable. If the frequency of collaboration is rare and can be limited to a fixed number of participants, it may be the right choice. Most reporting will focus on the primary salesperson, working in the standard way. Skilled analysts will be called on for the occasional report, which calls for factoring in collaboration. If this is not satisfactory, however, a bridge table will be required.
Using a Bridge for Multi-Valued Dimensions
In lieu of simplifying a many-to-many relationship, a multi-valued dimension can be accommodated through the design of a special table called a bridge. This approach is much more flexible but brings with it the danger of double-counting. It is not possible to eliminate the risk of double-counting, but it can be limited. If a bridged solution is chosen, special attention must be given to the behavior of slowly changing dimensions, and it may be necessary to add yet another table to satisfy the quirks of some software products.
Bridging the Relationship
The multi-valued dimension problem can be solved by developing a group table that sits between the fact table and the dimension table. Instead of directly referring to the dimension
Part III
Figure 9-3
Allowing for two salespeople
PART III
Dimension Design
table, each fact will refer to a single group. The group table is called a bridge, because it links that fact table with the dimension table. Here s how it works: A group table contains a group_key and a foreign key column referring to the dimension in question. A row is added for each group member. If there are three members of the group, the group table will have three rows for that group. Each row shares the same group_key. The fact table and the bridge can be joined using the group_key; the bridge table is joined to the dimension in turn. If that sounds a bit abstract, an example should help. Figure 9-4 shows a table called sales_group, which bridges the relationship between order_facts and salesrep. The table order_facts contains a sales_group_key, which indicates the set of salespeople involved in a particular order. The table sales_group has one row for each member of the group. Each row has only two columns: the group_key and the salesrep_key. Suppose that two salespeople, Ann and Henry, work together to bring in an order. The salesrep table has rows for Ann and Henry, as you can see in the instance table in the lower right of the diagram. In order to load the order into the fact table, a group is created for Ann and Henry assuming one does not already exist. The group is assigned a group_key. To make the example easy to read, a value of 1 is used, but the group_key has no intrinsic significance. There are two rows in the sales_group table for the collaboration between Ann and Henry. Each row contains the group identifier, 1, and salesrep_key of one of the participants: 222 for Ann and 333 for Henry. A row can now be loaded into the fact table for this group, holding the sales_group_key 1.
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