Database Reference
In-Depth Information
As shown in Figure 4-22 , type II dimensions are distinguished by tracking the changes using additional rows.
Each change adds a new row to the dimension table.
Figure 4-22. Adding type II slow changing dimension columns
Type III
In the SCD type III, the process is simplified by tracking only the current and previous values. If an additional
change occurs, the current value becomes a previous value, and the value before will be overwritten. Although
not as popular as type II, it is simple to implement and may be appropriate on occasion. In Figure 4-23 , the
column AuthorPreviousName tracks the previous name, and the ChangeDate column tracks the date the change
occurred.
Figure 4-23. A type III slow-changing dimension
The distinguishing characteristic of a type III SCD is that changes are tracked by adding columns, not
rows. So if you wanted to track the first, second, and third changes, you simply add columns for each version.
You might even give each column a name such as AuthorNameVersion2, AuthorNameVersion3, and so on.
The number of changes you track are determined by how many columns you add. Typically, though, it is most
common in type III to track only one change, which would be the previously used version (AuthorPreviousName
in this example).
 
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