Refactoring Nested Queries Into CTEs
A live-interview pattern: turning an unreadable nested query into stepwise CTEs.
The Live-Interview Refactor
A staple mid-level prompt: here is a query, make it readable. The interviewer hands you a deeply nested SELECT and watches how you decompose it. Turning nesting into a sequence of named CTEs is the cleanest answer.
This lesson walks the exact moves so you can do it calmly on a whiteboard.
Start With the Innermost Query
Nested subqueries execute conceptually from the inside out. So read the query inside-out too: find the deepest parenthesized SELECT first; that is your first pipeline stage.
Give it a descriptive name and lift it into a CTE. Everything that referenced that inner block now references the CTE name instead.
SELECT *
FROM (
SELECT customer_id, SUM(amount) AS total
FROM orders
GROUP BY customer_id
) t
WHERE t.total > 1000;All lessons in this course
- Writing Your First CTE
- Chaining Multiple CTEs
- CTE vs Subquery vs Temp Table
- Refactoring Nested Queries Into CTEs