SCD (Slowly Changing Dimension) is a very important concept in ETL and Data Warehouse in general. “Slowly Changing Dimension (SCD) is a dimension that stores and manages both current and historical data over time in a data warehouse” - Oracle This methodology is used to track changes in the dimensions that change slowly over time. Let’s have an example: If a customer in a bank has an account and the balance is changing every business day and this is the normal case that does not need an SCD technique. Then this customer decided to upgrade his account to a premium one. Our assumptions here: 1. The bank wants to track the balance and other changes starting from the opening of the basic account and do not want to open a new account but just an upgrade to premium. 2. Upgrading the account to a premium one force the bank to change the account number to meet the criteria of the premium accounts(let’s say the premium account must start with number 2 and the basic account must start with 1). 2