Part 80: Refactoring Nested Subqueries for Performance

Transforming slow row-by-row subqueries into high-performance set-based joins.

1. Subquery vs JOIN Transformation

-- ❌ Slow Correlated Subquery:
SELECT c.name, (SELECT MAX(o.total_amount) FROM orders o WHERE o.customer_id = c.id)
FROM customers c;

--  High-Performance GROUP BY JOIN:
SELECT c.name, MAX(o.total_amount)
FROM customers c
LEFT JOIN orders o ON c.id = o.customer_id
GROUP BY c.id, c.name;

Key Takeaways

• Convert correlated subqueries to joins and window functions for massive speedups.

In Part 81, we enter Module 9 to master Common Table Expressions (CTEs)!