advanced patterns with T-SQL INSERT...SELECT When Joining Multiple Tables - SQL Server 2019
I'm trying to configure I've spent hours debugging this and I'm trying to insert data from multiple tables into a new table using an `INSERT...SELECT` statement in SQL Server 2019, but I'm working with unexpected behavior with the results..... My goal is to combine data from a `Customers` table and an `Orders` table into a `CustomerOrders` table. Here's the query I wrote: ```sql INSERT INTO CustomerOrders (CustomerID, OrderID, OrderDate) SELECT c.CustomerID, o.OrderID, o.OrderDate FROM Customers c JOIN Orders o ON c.CustomerID = o.CustomerID WHERE o.OrderDate > '2022-01-01'; ``` After executing this, I noticed that the `CustomerOrders` table is getting populated with duplicate entries for some customers, even when I expected unique combinations of `CustomerID` and `OrderID`. I checked the `Customers` and `Orders` tables to see if there were duplicates, and both tables contain unique records. I also confirmed that `CustomerID` in the `Customers` table is a primary key. To troubleshoot, I ran the `SELECT` statement independently: ```sql SELECT c.CustomerID, o.OrderID, o.OrderDate FROM Customers c JOIN Orders o ON c.CustomerID = o.CustomerID WHERE o.OrderDate > '2022-01-01'; ``` This returned 10 rows, which is as expected, but when I check the `CustomerOrders` table, it shows 15 rows. I suspect it might be related to how the join is working, but I need to figure out why duplicates are being introduced during the insert. I've also tried using a `DISTINCT` clause in my `SELECT` query but it hasn't resolved the scenario: ```sql INSERT INTO CustomerOrders (CustomerID, OrderID, OrderDate) SELECT DISTINCT c.CustomerID, o.OrderID, o.OrderDate FROM Customers c JOIN Orders o ON c.CustomerID = o.CustomerID WHERE o.OrderDate > '2022-01-01'; ``` The result remains the same. Has anyone encountered this scenario before, or does anyone know why I might be getting these duplicates in my `CustomerOrders` table? For reference, this is a production desktop app. Any ideas how to fix this? Could someone point me to the right documentation?