Primary and foreign keys

Professional databases follow consistent patterns for connecting tables. Learning these patterns helps you identify join columns quickly. Databases use a pattern called primary key / foreign key (PK/FK) to organize relationships between tables. Understanding this pattern helps you know which columns to join on. Primary Key A primary key uniquely identifies each row in a table. No two rows can have the same primary key value. Common examples: Foreign Key A foreign key is a column that references the primary key of another table. It creates the link between tables: When you join tables, you typically connect a foreign key to its referenced primary key. This pattern appears in 70-80% of joins in real databases. The primary key and foreign key pattern is so universal in relational databases th

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This section is part of the Joins: 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.

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