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Explain how to update type II slowly changing dimensions providing examples, SQL code, and graphics

Slowly Changing Dimensions (SCDs) are commonly used in data warehousing to track and manage historical changes in dimension attributes over time. Type II SCDs keep a history of changes by creating new records for each change while maintaining a reference to the original record. I'll explain how to update Type II SCDs slowly, along with examples, SQL code, and graphics, similar to what we frequently do for our professional services customers.

Let's assume we have a dimension table called "Product" with the following structure:

To update the Type II SCD slowly, follow these steps:

Step 1: Insert new records for changes: When a change occurs in a dimension attribute, instead of updating the existing record, create a new record with the updated attribute values. Assign a new StartDate to the updated record and set the EndDate of the previous record to one day before the StartDate of the updated record.

For example, let's say we have an existing record for ProductID 1 with StartDate='2022-01-01' and EndDate='9999-12-31', and we want to update its Category from 'Electronics' to 'Appliances'. The SQL code for the insert would be:

This way, we maintain the historical data by creating a new record for the updated Category.

Step 2: Retrieve current data using the effective date: To retrieve the current data, you need to identify the records with an EndDate of '9999-12-31' or a future date. You can use the following SQL code to get the current data for a specific date:

This query will return the current data for the given date by selecting records that have a StartDate earlier or equal to the desired date and an EndDate greater than the desired date or '9999-12-31'.

Step 3: Visualize the changes: You can use a visualization tool or a graph to represent the changes in the Type II SCD over time. Here's a simple example showing the changes for the "Product" dimension:

This table visually represents the updates made to the "Category" attribute for ProductID 1 over time.

By following these steps and using the provided examples, SQL code, and graphics, you can effectively update Type II Slowly Changing Dimensions while preserving historical data.