In federated databases, where data is spread across multiple sources, managing slowly changing dimensions (SCD) can be a challenge. Slowly changing dimensions refer to data that changes over time but at a slower rate compared to other data. To effectively handle SCDs in federated databases, here are some techniques to consider:

1. Type 0 - Overwrite Existing Data (No History Tracking)

Sometimes, it may be acceptable to overwrite existing data without tracking its changes. This approach is suitable for dimensions where historical data is not necessary. Here’s an example of how you can implement Type 0 SCD in a federated database:

UPDATE dimension_table
SET attribute = 'new_value'
WHERE id = 'unique_id';

2. Type 1 - Overwrite Existing Data (Limited History Tracking)

Another approach is to overwrite existing data but keep a limited history of changes. This technique is referred to as Type 1 SCD. It involves updating the dimension table without creating additional records. Here’s an example of how to implement Type 1 SCD in a federated database:

UPDATE dimension_table
SET attribute = 'new_value',
    last_modified_date = CURRENT_TIMESTAMP
WHERE id = 'unique_id';

3. Type 2 - Insert New Records (Full History Tracking)

Type 2 SCD involves inserting new records for each change in the dimension data, effectively creating a full history. This approach enables analysis of data changes over time. Here’s an example of how to implement Type 2 SCD in a federated database:

INSERT INTO dimension_table
VALUES ('new_unique_id', 'attribute_value', CURRENT_TIMESTAMP);

4. Type 3 - Add Additional Columns for Limited History

Type 3 SCD requires adding additional columns to the dimension table to hold limited history. This technique is suitable for scenarios where tracking a limited number of changes is sufficient. Here’s an example of how to implement Type 3 SCD in a federated database:

ALTER TABLE dimension_table
ADD COLUMN attribute_prev_value ATTRIBUTE_TYPE,
ADD COLUMN attribute_prev_modified_date DATE;

5. Type 4 - Separate Historical and Current Data

Type 4 SCD involves creating separate tables to store historical and current data. Historical data is retained in one table, while the current data is stored in another. This approach provides a clear separation of historical and current information. Here’s an example of how to implement Type 4 SCD in a federated database using two tables:

CREATE TABLE historical_dimension_table
(
    id PRIMARY KEY,
    attribute ATTRIBUTE_TYPE,
    modified_date DATE
);

CREATE TABLE current_dimension_table
(
    id PRIMARY KEY,
    attribute ATTRIBUTE_TYPE
);

Handling slowly changing dimensions in federated databases requires careful consideration of the specific needs and constraints of your data. Depending on the requirements, one or a combination of these techniques can be used to effectively manage and track changes in dimensional data.

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