Primary Keys: Data Identity

Concepts covered: dmPrimaryKeys

A primary key is a column (or set of columns) that uniquely identifies each row. It is the minimum contract every table must fulfill: no two rows can represent the same entity. The primary key is not just a constraint. It is the mechanism by which every other operation in the relational model is made safe. Joins reference it. Foreign keys point to it. Deduplication logic anchors on it. If you choose it poorly, the damage ripples through every downstream query. What Makes a Good Primary Key Composite Primary Keys A composite PK uses two or more columns together to identify a row. They are the natural fit for junction tables in many-to-many relationships. An enrollment table linking students to courses has a composite PK of (student_id, course_id). The combination is unique even though neith

About This Interactive Section

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.

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.