Slowly Changing Dimensions

Concepts covered: paScdPipeline

What They Want to Hear 'SCD Type 2 keeps a full history of changes. When a value changes, I close the current row by setting an end date, and insert a new row with the updated value and a new start date. This means I can always answer: what was the customer's address when they placed that order last March?' That is the answer. SCD is about preserving history so you can join facts to the dimension values that were true at the time.

About This Interactive Section

This section is part of the Keeping Data Fresh: Beginner lesson on DataDriven, a free data engineering interview prep platform. Each section includes explanations, worked examples, and hands-on code challenges that execute in real time. SQL queries run against a live PostgreSQL database. Python runs in a sandboxed Docker container. Data modeling problems validate against interactive schema canvases. All content is framed around what data engineering interviewers actually test at companies like Meta, Google, Amazon, Netflix, Stripe, and Databricks.

How DataDriven Lessons Work

DataDriven combines four interview rounds (SQL, Python, Data Modeling, Pipeline Architecture) with adaptive difficulty and spaced repetition. Easy problems get harder as you improve. Weak concepts resurface until you master them. Your readiness score tracks progress across every topic interviewers test. Every lesson section ends with problems you solve by writing and running real code, not by picking multiple-choice answers.