4. SQL for Data Engineers (Joins, Window Functions)

4.2 Joins That Explode

By Ravindra Bagale · Last updated: 24 October 2026

Why. Many-to-many joins turn 10,000 orders into 2,000,000 rows. Dashboards lie.

-- Fictional sketch: check counts before/after join
SELECT COUNT(*) FROM orders;
SELECT COUNT(*) FROM orders o
JOIN order_items i ON o.order_id = i.order_id;

How. If counts explode, fix keys or aggregate items before join.