LEFT JOIN ... WHERE Right Side IS NULL

Concepts covered: sqlNullInJoins

The same English question maps to three SQL forms. All three return the same rows on clean data. You should be able to write each of them in six lines without thinking. Knowing all three is the floor. Knowing which one to default to is what the interviewer is reading you for. Form 1: LEFT JOIN WHERE NULL Join the two tables with a LEFT JOIN. The LEFT JOIN keeps every row from the left side, attaching matching rows from the right side where they exist and NULLs where they do not. Filter the result to keep only the rows where the right side is NULL. Those are the left rows that did not match. Trace it. Suppose customers has Alice, Bob, Carol. Orders has rows for Alice and Bob, none for Carol. The LEFT JOIN produces three rows: Alice paired with her order, Bob paired with his order, Carol pai

About This Interactive Section

This section is part of the Anti-Join: 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 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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