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Overview of Dimensions (191)

原创 网络安全 作者:tsinglee 时间:2007-11-13 11:13:17 0 删除 编辑

A dimension defines hierarchical (parent/child) relationships between pairs of
columns or column sets. Each value at the child level is associated with one and only
one value at the parent level. A hierarchical relationship is a functional dependency
from one level of a hierarchy to the next level in the hierarchy. A dimension is a
container of logical relationships between columns, and it does not have any data
storage assigned to it.

The CREATE DIMENSION statement specifies:
■ Multiple LEVEL clauses, each of which identifies a column or column set in the
dimension
■ One or more HIERARCHY clauses that specify the parent/child relationships
between adjacent levels
■ Optional ATTRIBUTE clauses, each of which identifies an additional column or
column set associated with an individual level

The columns in a dimension can come either from the same table (denormalized) or
from multiple tables (fully or partially normalized). To define a dimension over
columns from multiple tables, connect the tables using the JOIN clause of the
HIERARCHY clause.

For example, a normalized time dimension can include a date table, a month table, and
a year table, with join conditions that connect each date row to a month row, and each
month row to a year row. In a fully denormalized time dimension, the date, month,
and year columns are all in the same table. Whether normalized or denormalized, the
hierarchical relationships among the columns need to be specified in the CREATE
DIMENSION statement.

维度
1. 维度用于定义两个列或两个列集合之间的层次关系。位于子级的一个值与且仅与唯一一个父级的值相关。
2. 一个维度对象 只是定义了数据列之间的逻辑关系,其中并不实际存储任何数据
3. 在 CREATE DIMENSION 语句中包含了:
a. 多个 LEVEL 子句,每个子句指定了维度中某一层对应的数据列或列集
b. 一个或多个 HIERARCHY 字句,用于指定相邻层之间的父/子关系
c. 可选的 ATTRIBUTE 字句,每个子句指定了与一个层次相关的附加属性所对应的数据列或列集
4. 一个维度中使用的数据列可以来自同一个表(反规范化的),也可以来自多个表(完全规范化或部分规范化).

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来自 “ ITPUB博客 ” ,链接:http://blog.itpub.net/10599713/viewspace-982100/,如需转载,请注明出处,否则将追究法律责任。

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