Degenerate dimensions （退化的维）

Degenerate dimensions （退化的维）不需要建立维度表，就用事实表的字段为维。
A degenerate dimension is a dimension which is so simple that it isn't worth creating its own
dimension table. For example, consider following the fact table:
product_id time_id payment_method customer_id store_id item_count dollars
55 20040106 Credit 123 22 3 $3.54
78 20040106 Cash 89 22 1 $20.00
199 20040107 ATM 3 22 2 $2.99
55 20040106 Cash 122 22 1 $1.18
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and suppose we created a dimension table for the values in the payment_method column:
payment_method
Credit
Cash
ATM
This dimension table is fairly pointless. It only has 3 values, adds no additional information, and
incurs the cost of an extra join.
Instead, you can create a degenerate dimension. To do this, declare a dimension without a table,
and Mondrian will assume that the columns come from the fact table.
<Cube name="Checkout">
<!-- The fact table is always necessary. -->
<Table name="checkout">
<Dimension name="Payment method">
<Hierarchy hasAll="true">
<!-- No table element here.
Fact table is assumed. -->
<Level name="Payment method"
column="payment_method" uniqueMembers="true" />
</Hierarchy>
</Dimension>
<!-- other dimensions and measures -->
</Cube>
Note that because there is no join, the foreignKey attribute of Dimension is not necessary,
and the Hierarchy element has no <Table> child element or primaryKey attribute.