Refactoring Nested Subqueries into CTEs

Concepts covered: sqlCte

CTEs are a power tool. The same tool that makes a 50-line query readable can produce a 500-line query that is unreadable for a different reason: too many tiny CTEs, each doing one tiny thing, with the actual logic distributed across the chain. The interviewer is checking whether you recognize the anti-patterns and refactor before they ship to production. The 20-CTE monster If a query has 20 CTEs, something has gone wrong. Either the query is solving a problem that should be decomposed across multiple queries or views, or the CTEs are too small (each doing one trivial transformation that could be folded into the next), or the author was nesting CTEs as a substitute for thinking about the data flow. The rule of thumb: 3 to 7 CTEs per query is normal; 8 to 12 is the upper end; past 15 the que

About This Interactive Section

This section is part of the CTEs (Common Table Expressions): Intermediate 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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