What is SCD? What are the types of SCD?

SCD stands for Slowly Changing Dimension, a concept in data warehousing that deals with how to handle changes in dimension attributes over time. The main types are:

1. SCD Type 1 – Overwrites old data with new data, no history kept.
2. SCD Type 2 – Creates a new record for each change, preserving full history with start/end dates or flags.
3. SCD Type 3 – Stores a limited history by adding extra columns for previous values (e.g., previous city).
4. SCD Type 4 – Uses a separate history table to store changes while keeping the current values in the main table.
5. SCD Type 6 – Combines features of Types 1, 2, and 3 for more complex scenarios.

These patterns help analysts answer questions like "What was the customer’s address in 2018?" or "How many times did a product’s price change?".