Handling Slowly Changing Dimensions

Introduction Key Definitions Types of Slowly Changing Dimensions Methods for Handling SCDs Best Practices FAQ

Introduction

Slowly Changing Dimensions (SCD) are a common challenge in data warehousing, where the attributes of a dimension can change over time. This lesson explores how to effectively manage and model these changes to maintain data integrity and analytical accuracy.

Key Definitions

Dimension: A structure that categorizes facts and measures in order to enable users to answer business questions.
Slowly Changing Dimension (SCD): A dimension that changes slowly over time, rather than changing on a regular schedule, such as daily or weekly.

Types of Slowly Changing Dimensions

Type 0: Passive Method - No changes are recorded.
Type 1: Overwrite - Old data is overwritten with new data.
Type 2: Historical Data - New records are created to maintain historical data.
Type 3: Limited History - Only the current and previous values are stored.

Methods for Handling SCDs

Here are the common methods for handling Slowly Changing Dimensions:

Type 2 Example

In Type 2, we create a new record for changes while maintaining historical records. A typical schema might include the following fields:


CREATE TABLE Customer (
    CustomerID INT PRIMARY KEY,
    CustomerName VARCHAR(255),
    Address VARCHAR(255),
    EffectiveDate DATE,
    ExpiryDate DATE,
    IsActive BOOLEAN
);

With Type 2, when a customer's address changes, a new record is created with updated details, and the old record's expiry date is set. This maintains the history of the customer data.

Best Practices

Always include a surrogate key for dimension tables to ensure uniqueness.
Use effective and expiry dates in Type 2 dimensions for clarity on record validity.
Consider performance implications when choosing your SCD type.
Regularly audit your dimensions to ensure data quality and integrity.

FAQ

What is the main purpose of handling Slowly Changing Dimensions?

The main purpose is to ensure that historical data remains accurate and accessible even as current data changes.

Which type of SCD should I use?

The choice of SCD type depends on the specific business requirements regarding data retention and analysis. Type 2 is commonly used for maintaining a full history.

How do I implement Type 2 SCD in SQL?

You can implement Type 2 SCD by inserting new records with updated attributes and setting the expiry date on the old records.