Nitin Gupta

The biggest risk after a LEFT JOIN? 👉 Putting a WHERE condition on the right table. This turns your LEFT JOIN into an INNER JOIN, and most people don't even notice. Example: SELECT * FROM customers c LEFT JOIN orders o    ON c.id = o.customer_id WHERE o.order_date > '2024-01-01'; 🤔 This looks like a LEFT JOIN… but that WHERE o.order_date removes all customers with no orders, killing the purpose of the left join. ✅ Fix: Move the filter into the JOIN condition: LEFT JOIN orders o    ON c.id = o.customer_id   AND o.order_date > '2024-01-01'; This is one of the most common SQL bugs. Have you seen this happen in production? 😄

A tiny SQL mistake that silently kills performance. ⚠️ This query looks fine but forces a full table scan: SELECT * FROM orders WHERE DATE(created_at) = '2024-12-10'; Why? Applying DATE() makes the filter non-sargable, so indexes/partitions can't be used. ✅ Use this instead: WHERE created_at >= '2024-12-10' AND created_at < '2024-12-11'; Small change → huge improvement. Have you caught this in production before?

RIGHT JOIN isn't wrong. It's just unnecessary in most production SQL. From my experience: 👉 LEFT JOIN + table swap covers almost everything 👉 INNER JOIN handles the rest 👉 RIGHT JOIN mostly adds confusion Every time I see a RIGHT JOIN in a query, I have to mentally reverse it. And that's where mistakes creep in. 🤯 Most teams quietly avoid it - not because it's incorrect, but because it feels risky and hard to read. Do you still use RIGHT JOIN anywhere?