Required vs Optional

Optionality: What Happens When the FK Is NULL? A required relationship means the FK cannot be NULL: every order MUST have a customer. An optional relationship means the FK can be NULL: an employee might not have a department yet. This distinction directly affects your queries. An INNER JOIN drops rows with NULL FKs. A LEFT JOIN preserves them but produces NULLs in the joined columns. In analytical models, the standard practice for optional FKs is to create an 'Unknown' member in the dimension table (e.g., customer_id = -1, name = 'Unknown'). Then the FK is never NULL. Every fact row joins successfully, and reports show 'Unknown' instead of silently dropping rows. The Unknown member is not a cosmetic convenience. It is a data integrity mechanism. Without it, an INNER JOIN to the dimension s

About This Interactive Section

This section is part of the Relationships 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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