SQL Server: Unexpected duplicate rows when using GROUP BY with a JOIN on multiple tables
This might be a silly question, but I've looked through the documentation and I'm still confused about This might be a silly question, but I'm working with an scenario in SQL Server 2019 where my query is unexpectedly returning duplicate rows despite using `GROUP BY`... I have two tables: `Orders` and `Customers`. The goal is to get a count of orders per customer. However, when I include an additional field from the `Customers` table in the `SELECT` clause, the result set contains duplicates. Here's the query I used: ```sql SELECT c.CustomerID, c.CustomerName, COUNT(o.OrderID) AS OrderCount FROM Customers c JOIN Orders o ON c.CustomerID = o.CustomerID GROUP BY c.CustomerID, c.CustomerName; ``` I've verified that the `CustomerID` and `CustomerName` are unique in the `Customers` table. However, when I run the query, I get multiple rows for some customers, and the count seems inflated as well. I even tried using `DISTINCT` like this: ```sql SELECT DISTINCT c.CustomerID, c.CustomerName, COUNT(o.OrderID) AS OrderCount FROM Customers c JOIN Orders o ON c.CustomerID = o.CustomerID GROUP BY c.CustomerID, c.CustomerName; ``` But that didn’t help either. The counts seem to be influenced by NULL values in the `Orders` table for some customers. I’ve also checked if there are any unexpected `NULL` values in the `CustomerID` of the `Orders` table, but they appear to be handled correctly. Is there a way to ensure that I get the correct count of orders without duplicates? Any suggestions on how to troubleshoot this further would be greatly appreciated! What's the best practice here? The project is a REST API built with Sql. Thanks in advance! This is happening in both development and production on CentOS. Any examples would be super helpful.