Composite Keys

Concepts covered: dmCompositeKeys

A composite key uses multiple columns together to uniquely identify a row. The most common use case is a junction table for a many-to-many relationship. An enrollment table linking students to courses has a composite PK of (student_id, course_id). Neither column is unique alone, but the combination is. Composite PK vs Surrogate PK in Junction Tables Neither approach is universally correct. If the junction table is purely a link (no extra attributes), a composite PK is cleaner. If the junction table becomes an entity in its own right (e.g., enrollments with grades and dates), add a surrogate key and keep the composite as a UNIQUE constraint. Composite Keys in Slowly Changing Dimensions In SCD Type 2, every historical version of a dimension row needs a unique identifier. One approach is a co

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This section is part of the Keys & Identity 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.

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