Correlated subqueries (EXISTS)

Concepts covered: sqlExists

Watch how each order probes the subquery: the outer row expands, the payments table responds, and only rows with a qualifying payment move into the result. Each outer row probes the subquery independently. As soon as a qualifying payment is found, the subquery stops scanning and the order passes through. Orders with no matching payment are excluded entirely. EXISTS Efficiency Which subquery approach checks for related rows more efficiently?

About This Interactive Section

This section is part of the Filtering: 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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