Database Reference
In-Depth Information
<
MeasureGroups
>
13
<
MeasureGroup name=
'
Sales
'
table=
'
Sales
'
>
14
<
Measures
>
15
...
16
<
/Measures
>
17
<
DimensionLinks
>
18
...
19
<
/DimensionLinks
>
20
<
/MeasureGroup
>
21
<
/MeasureGroups
>
22
<
/Cube
>
23
<
/Schema
>
24
The
Schema
element (starting in line 1) defines all other elements in the
schema. The
PhysicalSchema
element (lines 3-5) defines the tables that are
the source data for the dimensions and cubes, and the foreign key links
between these tables. The
Dimension
element (lines 6-8) defines the
shared
dimension
Time
, which is used several times in the Northwind cube for
the role-playing dimensions
OrderDate
,
DueDate
,and
ShippingDate
. Shared
dimensions can also be used in several cubes. The
Cube
element (lines 9-23)
defines the
Sales
cube. A cube schema contains dimensions and measures,
the latter organized in measure groups. The
Dimensions
element (lines 10-
12) defines the dimensions of the cube. The measure groups are defined
using the element
MeasureGroups
(lines 13-22). The measure group
Sales
(lines 14-21) defines the measures using the
Measures
element (lines 15-17).
The
DimensionLinks
element (lines 18-20) defines how measures relate to
dimensions. We detail next each of the elements introduced above.
5.10.1 Schemas and Physical Schemas
The
Schema
element is the topmost element of a cube schema. It is the
container for all its schema elements. A schema has a name and may have
other attributes such as description and the version of Mondrian in which it
is written. A schema always includes a
PhysicalSchema
element and one or
more
Cube
elements. Other common elements are
Dimension
(to define shared
dimensions) and
Role
for access control.
The
PhysicalSchema
element defines the
physical schema
,whichstates
the tables and columns in the database that provide data for the dimensions
and cubes in the data warehouse. The physical schema isolates the logical data
warehouse schema from the underlying database. For example, a dimension
can be based upon a table that can be a real table or an SQL query in the
database. Similarly, a measure can be based upon a column that can be a
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