Slowly Changing Dimensions (SCD) are a common challenge in data warehousing and database management. SCDs refer to dimensions that change over time, requiring careful handling to maintain the integrity and accuracy of the data.

There are different types of SCDs, with Type 3 being one of the most commonly used. Type 3 SCDs keep a limited history of changes by storing only the most recent value and the original value. This is useful when you need to track some changes but don’t require a complete historical record.

In this blog post, we will explore how to handle Type 3 SCDs in SQL. Let’s dive in!

1. Identifying Type 3 SCD Columns

To handle Type 3 SCDs, you first need to identify the columns that require tracking changes. These columns are typically the ones that are subject to updates.

For example, let’s consider a table called customers with the following columns:

CREATE TABLE customers (
    id INT PRIMARY KEY,
    name VARCHAR(50),
    email VARCHAR(100),
    status VARCHAR(10),
    last_updated DATETIME
);

In this case, the status column could be a Type 3 SCD column. We want to track its changes while keeping a limited history.

2. Adding Additional Columns

To handle Type 3 SCDs, you’ll need to add additional columns to the table. These columns will store the original and most recent values.

In our example, we can add two columns: status_original and status_latest:

ALTER TABLE customers ADD COLUMN status_original VARCHAR(10);
ALTER TABLE customers ADD COLUMN status_latest VARCHAR(10);

Now we have the necessary columns to track changes.

3. Updating the Table

Whenever a change in the SCD column occurs, we need to update both the status_latest and last_updated columns, as well as preserve the original value in the status_original column.

UPDATE customers
SET status_original = status_latest,
    status_latest = 'NEW_STATUS',
    last_updated = now()
WHERE id = 1;

In this example, we’re updating the status_latest column to a new value (‘NEW_STATUS’), preserving the original value in status_original, and updating the last_updated column with the current date and time. The WHERE clause restricts the update to a specific row.

4. Querying the Changes

To analyze the changes in Type 3 SCDs, you can write queries that compare the original and latest values.

SELECT id, name, status_original, status_latest, last_updated
FROM customers
WHERE id = 1;

This query retrieves the original and latest values of the status column for a specific customer.

Conclusion

Handling Type 3 Slowly Changing Dimensions in SQL requires the identification of SCD columns, the addition of additional columns to store original and latest values, and proper update and query operations. By following these steps, you can effectively track and manage changes in your SCDs.

#datawarehousing #SQL