Three-valued logic (NOT IN)

SQL uses three-valued logic (TRUE, FALSE, UNKNOWN) instead of classical boolean logic. This fundamentally changes how compound conditions behave and breaks some intuitive rules. Three-Valued Logic Boolean operations in SQL don't just have TRUE and FALSE outcomes; UNKNOWN is a third possibility that propagates through logical operators. Truth Tables with UNKNOWN Common Pitfalls Three-valued logic creates counterintuitive behaviors that cause subtle bugs even for experienced developers. The NOT IN Trap De Morgan's Laws Break Down The Excluded Middle

About This Interactive Section

This section is part of the NULL Values: Advanced 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.