👀 Views: 55 💬 Answers: 1 📅 Created: 2025-06-02

PostgreSQL SQL LEFT JOIN

I'm migrating some code and I'm working with an scenario with a LEFT JOIN query in PostgreSQL where I expect to get a count of related records, but the result set is not what I anticipated. My database consists of two tables: `users` and `orders`. Each user can have multiple orders, and I want to list all users along with the count of their orders. Here's the SQL query I'm using: ```sql SELECT u.id, u.name, COUNT(o.id) AS order_count FROM users u LEFT JOIN orders o ON u.id = o.user_id GROUP BY u.id, u.name; ``` When I run this query, I get correct counts for users who have orders, but for users without any orders, the `order_count` shows as `0`. However, I also noticed that the query is returning duplicate rows for users who have multiple orders. I expected to see each user listed only once with their order count. After some debugging, I tried modifying the query by removing the `GROUP BY` clause and using the aggregate function directly: ```sql SELECT u.id, u.name, COUNT(o.id) AS order_count FROM users u LEFT JOIN orders o ON u.id = o.user_id; ``` But this resulted in an behavior: `behavior: column "u.id" must appear in the GROUP BY clause or be used in an aggregate function`. It seems like I need both the grouping and the counting to work together properly. I also checked the version of PostgreSQL I'm using, which is 12.5, and I've ensured that both tables are indexed properly on the join columns to optimize performance. I’m confused about how to correctly structure the query to get a unique list of users along with the correct order counts without duplicates. Any advice on how to solve this scenario would be greatly appreciated! This is happening in both development and production on CentOS. For reference, this is a production application. I appreciate any insights!