What are the different types of slowly changing dimension (SCD) models?
In data warehousing and business intelligence, slowly changing dimensions (SCD) are a critical concept that deals with how to manage changes over time in dimension tables. These tables represent entities like customers, products, or employees, and their attributes change slowly over time. There are several types of SCD models, each designed to handle changes in different ways. Understanding these types is essential for designing robust and efficient data warehouses.
1. Type 1 SCD: Overwrite
The simplest form of SCD is Type 1, which simply overwrites the existing attribute values with the new ones. When a change occurs, the entire record is updated to reflect the new information. This approach is straightforward but can lead to data loss if historical information is important.
2. Type 2 SCD: Add New Rows
Type 2 SCD handles changes by adding new rows to the dimension table for each change, while keeping the original row intact. This allows for the preservation of historical data. For example, if a customer changes their address, a new row is created with the new address, and the original row remains with the old address. This type of SCD is useful when the dimension table needs to retain historical data for analysis.
3. Type 3 SCD: Add Attributes
Type 3 SCD is used when the attribute being changed is not critical to the business logic and does not require the retention of historical data. In this model, new columns are added to the dimension table to store the historical values of the attribute. For instance, if a product’s price changes, a new column is added to store the new price, and the old price is retained in the existing column.
4. Type 4 SCD: Complex SCD
Type 4 SCD is a more advanced model that combines aspects of Type 2 and Type 3 SCD. It is used when a dimension table requires complex handling of changes, such as merging multiple records or splitting a single record into multiple records. This type of SCD is often used in scenarios where business rules dictate that certain actions must be taken when specific changes occur.
5. Type 5 SCD: Temporal SCD
Type 5 SCD is a variation of Type 2 SCD that adds a time dimension to the dimension table. This allows for the tracking of historical changes over time, making it possible to analyze the state of a dimension at any point in history. Temporal SCD is particularly useful for compliance and audit purposes.
In conclusion, the different types of SCD models provide various approaches to handling changes in dimension tables over time. Each type has its advantages and use cases, and understanding these models is crucial for designing a data warehouse that can effectively support business intelligence and reporting needs.
