Why should you care?

An order with CustomerId = 99999 when no such customer exists breaks reports and refunds.

See it live — copy this example

Run in SQL Server Management Studio (SSMS) or Azure Data Studio.

USE DataVerse;
-- This fails if CustomerId 99999 does not exist:
BEGIN TRY
    INSERT INTO dbo.Orders (City, Amount, CustomerId)
    VALUES (N'Goa', 100, 99999);
END TRY
BEGIN CATCH
    SELECT ERROR_MESSAGE() AS ErrMsg;
END CATCH;

SELECT o.OrderId, o.CustomerId
FROM dbo.Orders o
LEFT JOIN dbo.Customers c ON c.CustomerId = o.CustomerId
WHERE c.CustomerId IS NULL; -- should be empty when integrity holds

What happened?

• The TRY/CATCH shows the FK violation message.
• The LEFT JOIN check finds orphan orders if someone disabled constraints — a health query.

Practice next

1. Attempt the bad INSERT and read the error.
2. Insert with a real CustomerId.
3. Run the orphan check query — expect no rows.
4. Add FK from OrderItems.ProductId to Products.
5. Query sys.foreign_keys for DataVerse.

Remember

FKs keep child keys valid. Orphan rows are a data bug. Catch violations early at write time.

No orphan orders

DataVerse Orders.CustomerId FK enforces real customers.

Outcome: Finance reports never show “unknown customer” gaps from bad IDs.