👀 Views: 88 💬 Answers: 1 📅 Created: 2025-06-11

MySQL subquery GROUP BY SQL

I'm prototyping a solution and I'm converting an old project and I've looked through the documentation and I'm still confused about I'm working with an scenario with MySQL 5.7 where a subquery in my SELECT statement seems to produce unexpected results when combined with GROUP BY... The subquery is supposed to return a count of related records, but the values appear inconsistent with what I expect. Here’s a simplified example of my query: ```sql SELECT a.id, a.name, (SELECT COUNT(*) FROM orders o WHERE o.user_id = a.id) as order_count FROM users a GROUP BY a.id, a.name; ``` When I run this query, the `order_count` sometimes shows values that seem to be aggregated incorrectly. For instance, if a user has multiple orders, the count should reflect that, but in some cases, it just returns `1` or even `0`. I’ve made sure that there are indeed multiple entries in the `orders` table for some users, so I need to figure out why the result is off. I've tried changing the subquery to a JOIN instead: ```sql SELECT a.id, a.name, COUNT(o.id) as order_count FROM users a LEFT JOIN orders o ON o.user_id = a.id GROUP BY a.id, a.name; ``` This variation seems to work correctly and returns the expected counts. However, I still want to understand why the original subquery approach didn't yield the correct results. Is there a specific behavior in MySQL when dealing with GROUP BY and subqueries that I might be overlooking? Any insights or best practices would be greatly appreciated! What's the correct way to implement this? I'm working with Sql in a Docker container on Ubuntu 20.04.