IBM Cognos Dynamic Cubes

Chapter 4. Modeling dynamic cubes 79

4.5.1 Using relative time

Cognos Dynamic Cubes generate built-in relative time members:

򐂰 Current period

򐂰 Prior period

򐂰 Period to date

򐂰 Prior period to date

򐂰 Period to date change

򐂰 Period to date growth

If you do not set the current period value for a level, the last member in the level will be

deemed to be the current period.

Instructions for creating a time dimension are in 4.14, “Creating a time dimension” on

page 112.

4.6 Slowly changing dimensions

Slowly changing dimensions (SCD) are a type of dimension in which the data describing the

entities in a dimension will change over time. This section describes the slowly changing

dimension types and use cases that Dynamic Cubes supports.

4.6.1 Modeling slowly changing dimensions

An example t of a slowly changing dimension that is easy to understand is an employee

dimension. Employees are hired, promoted, given raises, change positions, change

workplace locations, resign, retire, and are let go. The effect of these changes can alter the

structure of a tree that describes the hierarchical relationship between members of the

organization. For example, a change in position might result in an employee becoming a

manager and acquiring subordinates who then appear under the employee in the member

tree. The change can result in an employee exchanging one set of subordinates for another

set.

How the data changes are handled classify the types of slowly changing dimensions.

Although the types and classifications of slowly changing dimension types vary, the definition

of Type I and Type II SCDs seem to be universally recognized.

򐂰 Type I SCDs are characterized by the overwriting of the attribute state by the new data.

This type, obviously, has drawbacks.

򐂰 Type II SCDs are characterized by a new record being created in the table for each

change in the attributes of an entity tracked by the table. Slowly changing dimensions of

this type need some way to identify each record in the dimension table to distinguish one

member instance from another. The natural key of an entity will be repeated but a

surrogate key will be created to uniquely identify each record.

Cognos Dynamic Cubes supports Type II slowly changing dimensions.