Data Warehouse — Slowly Changing Dimensions (SCDs)

 Slowly Changing Dimensions (SCDs) are a concept in data warehousing used to manage and track changes in dimension data over time. Dimension tables typically contain descriptive attributes related to the facts in a data warehouse, such as customer or product information. However, these attributes can change over time, and different strategies are required to handle these changes while maintaining historical accuracy and data integrity.

A Slowly Changing Dimension (SCD) is a dimension that stores and manages both current and historical data over time in a data warehouse. It is considered and implemented as one of the most critical ETL tasks in tracking the history of dimension records. Here are some examples of SCDs:

* Customer dimension: Attributes like name, address, phone number, age, gender, and location

* Product dimension: Attributes like price, category, specifications, and descriptions

* Employee dimension: Attributes like job title, department, and salary

Types of Slowly Changing Dimensions (SCDs) — There are several types of SCDs, each with different approaches to handling changes:

• SCD Type 0 (Fixed Dimension)
  — Definition: No changes are allowed after the initial insert. The data remains as it was at the time of insertion.
  — Use Case: Suitable for historical data that should never change, such as product serial numbers or other immutable records.
• SCD Type 1 (Overwrite)
  — Definition: Updates the dimension record by overwriting the old data with the new data. No historical data is kept.
  — Use Case: Use when you don’t need to maintain historical changes, and only the current data is relevant. For example, correcting a typo in a customer’s name.
• SCD Type 2 (Add New Row)
  — Definition: Creates a new record for each change, keeping a full history of changes over time. Typically, this includes an additional column for tracking the effective date or current status.
  — Use Case: Use when it’s important to track the historical changes of a dimension over time, such as tracking a customer’s address changes.
  — Example: A customer moves to a new address. A new record with the new address is added, and the old record is marked as expired.
• SCD Type 3 (Add New Attribute)
  — Definition: Adds new columns to the existing dimension table to track changes. Typically, the table has columns for the current value and one or more previous values.
  — Use Case: Use when you need to keep only a limited history of changes, such as keeping track of the previous address of a customer.
  — Example: If a company reorganizes its departments, the old department name might be kept in a separate column while the current department name is updated.
• SCD Type 4 (History Table)
  — Definition: Uses a separate historical table to keep track of all changes to a dimension record, while the main dimension table keeps only the current data.
  — Use Case: Use when you want to maintain a clear separation between current and historical data, often for performance reasons.
• SCD Type 6 (Hybrid SCD)
  — Definition: Combines aspects of SCD Type 1, Type 2, and Type 3 to capture current, historical, and limited historical data in the same table. This type involves updating the existing record, adding a new record, and keeping a column for the previous value.
  — Use Case: Use when a combination of current, historical, and previous data is required. For example, tracking both the current and previous addresses of a customer while keeping a history of all changes.

Understanding the Types

- Choosing the Right Type: The type of SCD used depends on the specific requirements of the data warehouse and how important historical data is for the business processes.

- Trade-offs: Each type has its trade-offs in terms of storage, complexity, and performance. SCD Type 1 is simple but loses historical data, while SCD Type 2 provides a complete history but increases storage and complexity.

Understanding SCDs is crucial for designing a data warehouse that accurately reflects historical data while meeting the needs of the business for current and past information.

SCD is not a methodology. It is a way to describe the data as it being presented in a table. This really depends on the requirements of the business.

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