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From the data model point of view, each line of an order might refer to more than one reason.
Therefore, you cannot add a SalesReasonID column to the SalesOrderDetail table because, when
you do that, you can define only a single reason for each line, whereas your data model requires
a variable number of reasons for each single line.
You can clearly see the situation in Figure 4-32. If you set up a relationship between
FactInternetSales and DimSalesReason, you can slice sales by sales reason, but you are
limited to only one sales reason for each order line.
FIguRE 4-32 The wrong way to model sales reasons.
To set up the relationship correctly, you need an additional table. If you add an intermediate
table that contains an order line and a sales reason, you can use this additional table to define
both many sales reasons for a single line of an order and many lines of orders for a single sales
reason. This is why these kinds of relationships are called many-to-many: if you need to relate
many rows in a table with many rows in another one, you can do that using an intermediate
table. This intermediate table acts as a bridge between the two original tables: therefore, it is
commonly called a bridge table .
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