Foreign Keys

Concepts covered: dmForeignKeys

A foreign key is a column in one table that references the primary key of another. It is how relational databases enforce referential integrity: every FK value must either match an existing PK in the parent table, or be NULL. Without FK enforcement, your tables can accumulate orphaned rows that reference customers, products, or orders that no longer exist. Cloud Data Warehouses Do Not Enforce FKs This is one of the most important things to know as a data engineer. Redshift, BigQuery, Snowflake, and Databricks all let you declare FK constraints but do not enforce them at write time. You can insert an order with customer_id = 999 even if no customer 999 exists. The constraint is purely informational, used by the query optimizer for join planning. Cascade Behavior The FK Design Principle Thre

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.

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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.