CTEs vs nested subqueries (the WITH clause) | SQL interview

Nested subqueries are write-only code. CTEs make the same logic readable.

Common Table Expressions, the WITH clause. You name each step as its own block, then build on it, so the query reads top to bottom instead of inside out.

A CTE is a named temporary result you reference later. It doesn't change what runs, it restructures it so each transformation is a labeled step you (and the next engineer) can follow.

How it works:
- WITH step1 AS (...), step2 AS (SELECT ... FROM step1) ...
- Each CTE is a named block you can reference downstream
- Chain them so logic flows top to bottom, not nested inside out
- Great for multi-step transforms and reusing a result twice

The gotcha: a CTE referenced many times may be recomputed each time (engine-dependent), so for a heavy block used a lot, a temp table can be faster. Readability versus performance.

CTEs for readability, or subqueries the optimizer may handle better?

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