My query checked 1M rows... 1M times. I didn't know correlated subqueries did this. 💀 What I wrote: SELECT customer_id, name, (SELECT COUNT(*) FROM orders o WHERE o.customer_id = c.customer_id) as order_count FROM customers c; -- ⏱️ 3 minutes 47 seconds What actually happened: For EACH customer → Run the subquery 1M customers = 1M subqueries Database: melting The fix: SELECT c.customer_id, c.name, COUNT(o.order_id) as order_count FROM customers c LEFT JOIN orders o ON c.customer_id = o.customer_id GROUP BY c.customer_id, c.name; -- ⏱️ 1.2 seconds The rule: Correlated subquery = Loop in disguise JOIN + GROUP BY = Run once 190x faster. Same data. How many correlated subqueries are hiding in your code right now? 👀 #SQL #Performance #DataEngineering #SQLTips