When you step through the Slowly Changing Dimensions Wizard, you choose the columns in which you want to detect changes. The Designer creates an additional column for each column you select and names the additional column after the original, PM_PREV_

ColumnName

. The Integration Service uses these columns to store previous dimension values.

When the Integration Service writes a new dimension to the target, the

previous

columns remain null. Each time the Integration Service updates a dimension, it writes existing data into the corresponding

previous

column, and then writes updates into the original column. As a result, each row in a Type 3 Dimension target contains current dimension data. Each row also contains the previous versions of dimension data, if the dimension has changed.

For example, the first time the Integration Service writes the following dimensions to the target, the

previous

column, PM_PREV_STYLES remains null:

When the Integration Service updates these rows, it writes the values in the STYLES column (20 and 25) into PM_PREV_STYLES, and then replaces the style data with new style data (14 and 31):