Slowly Changing Dimensions (SCD) are techniques used in data warehousing to manage changes in dimension data over time. They determine how updates to records, such as customer addresses or product information, are stored while maintaining the required level of historical data.
In simple terms, SCD methods define whether old data is overwritten, preserved, or partially retained when changes occur.
SCD Type 1
Type 1 updates the existing record by replacing the old value with the new one.
Characteristics:
- Overwrites existing data
- Does not keep historical records
- Simple and fast to implement
- Best for correcting errors or updating non-historical information
SCD Type 2
Type 2 preserves history by creating a new record whenever a change occurs. The old record remains in the database, often with effective dates or a version number.
Characteristics:
- Maintains complete historical data
- Creates a new row for each change
- Supports historical reporting and trend analysis
- Commonly used in data warehouses
SCD Type 3
Type 3 stores both the current value and a limited amount of previous information within the same record.
Characteristics:
- Keeps limited history
- Adds new columns for previous values
- Suitable when only recent changes need to be tracked
- Uses less storage than Type 2
When Should Each Type Be Used?
- Type 1: When historical data is not needed and only the latest value matters.
- Type 2: When complete history must be preserved for reporting, auditing, or compliance.
- Type 3: When only the current and previous values need to be retained.
Choosing the appropriate SCD type depends on business requirements and how historical information will be used.
Conclusion
Slowly Changing Dimensions (SCD) help manage changes to dimension data in a data warehouse. Type 1 overwrites existing data, Type 2 preserves complete history by creating new records, and Type 3 stores limited historical information in the same record. Selecting the right approach ensures accurate reporting, efficient storage, and effective historical analysis.