Source: http://www.folkstalk.com/2012/03/slowly-changing-dimensions-scd-types.html
Slowly changing dimensions are the dimensions in which the data changes slowly, rather than changing regularly on a time basis.
For example, you may have a customer dimension in a retail domain. Let say the customer is in India and every month he does some shopping. Now creating the sales report for the customers is easy. Now assume that the customer is transferred to United States and he does shopping there. How to record such a change in your customer dimension?
You could sum or average the sales done by the customers. In this case you won't get the exact comparison of the sales done by the customers. As the customer salary is increased after the transfer, he/she might do more shopping in United States compared to in India. If you sum the total sales, then the sales done by the customer might look stronger even if it is good. You can create a second customer record and treat the transferred customer as the new customer. However this will create problems too.
Handling these issues involves SCD management methodologies which referred to as Type 1 to Type 3. The different types of slowly changing dimensions are explained in detail below.
SCD Type 1: SCD type 1 methodology is used when there is no need to store historical data in the dimension table. This method overwrites the old data in the dimension table with the new data. It is used to correct data errors in the dimension.
As an example, i have the customer table with the below data.
surrogate_key customer_id customer_name Location
------------------------------------------------
1 1 Marspton Illions
Here the customer name is misspelt. It should be Marston instead of Marspton. If you use type1 method, it just simply overwrites the data. The data in the updated table will be.
surrogate_key customer_id customer_name Location
------------------------------------------------
1 1 Marston Illions
The advantage of type1 is ease of maintenance and less space occupied. The disadvantage is that there is no historical data kept in the data warehouse.
SCD Type 3: In type 3 method, only the current status and previous status of the row is maintained in the table. To track these changes two separate columns are created in the table. The customer dimension table in the type 3 method will look as
surrogate_key customer_id customer_name Current_Location previous_location
--------------------------------------------------------------------------
1 1 Marston Illions NULL
Let say, the customer moves from Illions to Seattle and the updated table will look as
surrogate_key customer_id customer_name Current_Location previous_location
--------------------------------------------------------------------------
1 1 Marston Seattle Illions
Now again if the customer moves from seattle to NewYork, then the updated table will be
surrogate_key customer_id customer_name Current_Location previous_location
--------------------------------------------------------------------------
1 1 Marston NewYork Seattle
The type 3 method will have limited history and it depends on the number of columns you create.
SCD Type 2: SCD type 2 stores the entire history the data in the dimension table. With type 2 we can store unlimited history in the dimension table. In type 2, you can store the data in three different ways. They are
Versioning
Flagging
Effective Date
SCD Type 2 Versioning: In versioning method, a sequence number is used to represent the change. The latest sequence number always represents the current row and the previous sequence numbers represents the past data.
As an example, let’s use the same example of customer who changes the location. Initially the customer is in Illions location and the data in dimension table will look as.
surrogate_key customer_id customer_name Location Version
--------------------------------------------------------
1 1 Marston Illions 1
The customer moves from Illions to Seattle and the version number will be incremented. The dimension table will look as
surrogate_key customer_id customer_name Location Version
--------------------------------------------------------
1 1 Marston Illions 1
2 1 Marston Seattle 2
Now again if the customer is moved to another location, a new record will be inserted into the dimension table with the next version number.
SCD Type 2 Flagging: In flagging method, a flag column is created in the dimension table. The current record will have the flag value as 1 and the previous records will have the flag as 0.
Now for the first time, the customer dimension will look as.
surrogate_key customer_id customer_name Location flag
--------------------------------------------------------
1 1 Marston Illions 1
Now when the customer moves to a new location, the old records will be updated with flag value as 0 and the latest record will have the flag value as 1.
surrogate_key customer_id customer_name Location Version
--------------------------------------------------------
1 1 Marston Illions 0
2 1 Marston Seattle 1
SCD Type 2 Effective Date: In Effective Date method, the period of the change is tracked using the start_date and end_date columns in the dimension table.
surrogate_key customer_id customer_name Location Start_date End_date
-------------------------------------------------------------------------
1 1 Marston Illions 01-Mar-2010 20-Fdb-2011
2 1 Marston Seattle 21-Feb-2011 NULL
The NULL in the End_Date indicates the current version of the data and the remaining records indicate the past data.
