Slowly Changing Dimensions (SCD) are used in data warehousing to manage and track changes to dimension data over time. Here are the three main types of SCDs with simple examples:
SCD Type 1
Purpose: Overwrites old data with new data. It doesn't keep any history of changes. Example:
Imagine you have a customer table.
A customer's address changes from "123 Old St" to "456 New St".
In SCD Type 1, you simply update the address to "456 New St" and lose the previous address.
SCD Type 2
Purpose: Maintains full historical data by adding new records when changes occur. Example:
Using the same customer table.
When the address changes, instead of updating the existing record, a new record is added.
Each record will have a version number or effective dates to indicate the time period it was active.
| Customer ID | Name | Address | Version | Effective Date |
|---|---|---|---|---|
| 1 | John Doe | 123 Old St | 1 | 2020-01-01 |
| 1 | John Doe | 456 New St | 2 | 2022-01-01 |
SCD Type 3
Purpose: Tracks changes using additional columns to store historical data for certain attributes. Example:
When the customer's address changes, instead of creating a new record, a new column is added to store the previous address.
| Customer ID | Name | Current Address | Previous Address |
|---|---|---|---|
| 1 | John Doe | 456 New St | 123 Old St |
These methods help manage the evolution of dimensional data over time, allowing for accurate historical reporting and analysis
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