SCD Type 3 Implementation using Informatica PowerCenter

J Cyriac | Jan 6, 2013 | ETL Design | Mapping Tips

Unlike SCD Type 2, Slowly Changing Dimension Type 3 preserves only few

history versions of data, most of the time 'Current' and Previous' versions. The

'Previous' version value will be stored into the  additional columns with in the same dimension

record.  In this article lets discuss the step by step implementation of SCD Type 3 using

Informatica PowerCenter.

The number of records we store in SCD Type 3 do not increase exponentially as we do not insert

a record for each and every historical record. Hence we may not need theperformance

improvement techniques used in the SCD Type 2 Tutorial.

Understand the Staging and Dimension Table.

For our demonstration purpose, lets consider the CUSTOMER Dimension. Here we will keep

previous version of CITY, STATE, ZIP into its corresponding PREV columns. Below are the

detailed structure of both staging and dimension table.

Staging Table

In our staging table, we have all the columns required for the dimension table attributes. So no

other tables other than Dimension table will be involved in the mapping. Below is the structure of

our staging table.

Key Points 

1. Staging table will have only one days data.

2. Data is uniquely identified using CUST_ID.

3. All attribute required by Dimension Table is available in the staging table

Dimension Table

Here is the structure of our Dimension table.

Key Points

1. CUST_KEY is the surrogate key.

2. CUST_ID is the Natural key, hence the unique record identifier.

3. Previous versions are kept in PREV_CITY, PREV_STATE, PREV_ZIP columns.

Mapping Building and Configuration

Step 1

Lets start the mapping building process. For that pull the CUST_STAGE source definition into

the mapping designer.

Step 2

Now using a LookUp Transformation  fetch the existing Customer columns from the dimension

table T_DIM_CUST. This lookup will give NULL values if the customer is not already existing in

the Dimension tables. 

LookUp Condition : IN_CUST_ID = CUST_ID

Return Columns : CUST_KEY, CITY, STATE, ZIP

Step 3

Using an Expression Transformation to identify the records for Insert and Update using below

expression. Additionally, map the columns from the LookUp Transformation to the Expression as

shown below.  With this we get both  the previous and current values of the CUST_ID.

o CREATE_DT :- SYSDATE

o UPDATE_DT :- SYSDATE

o INS_UPD :- IIF(ISNULL(CUST_KEY),'INS', IIF(CITY <> PREV_CITY OR STATE

<> PREV_STATE OR ZIP <> PREV_ZIP, 'UPD'))   

Note : If in case there are too many columns to be compared  to build the INS_UPD logic, make

use of CheckSum Number  (MD5() Function) to make it simple.       

Step 4

Map the columns from the Expression Transformation to a Router Transformation and create two

groups (INSERT, UPDATE) in Router Transformation using the below expression. The mapping

will look like shown in the image.

o INSERT :- IIF(INS_UPD='INS',TRUE,FALSE)

o UPDATE :- IIF(INS_UPD='UPD',TRUE,FALSE)

INSERT Group

Step 5

Every records coming through the 'INSERT Group' will be inserted into the Dimension table

T_DIM_CUST.

Use a Sequence generator transformation to generate surrogate key CUST_KEY as shown in

below image. And map the columns from the Router Transformation to the target. Leave all

'PREV' columns unmapped as shown below image.

Note : Update Strategy is not required, if the records are set for Insert.

UPDATE Group

Step 6

Records coming from the 'UPDATE Group' will update the customer Dimension with Current

customer attributes and the 'PREV' attributes. Add an Update Strategy Transformation before the

target instance and set it as DD_UPDATE. Below is the structure of the mapping.

We are done with the mapping building and below is the structure of the completed mapping.

Workflow and Session Creation

There is not any specific properties required to be given during the session configuration. 

Below is a sample data set taken from the Dimension table T_DIM_CUST. See the highlighted

values.