There are several cases in SQL where it's necessary to update a variable once per row in a result, accumulating all of the changes together.

A sample of code that works in SQL Server:

DECLARE @InvoicesList VARCHAR(1024) 
 
SELECT @InvoicesList = COALESCE(@InvoicesList + ', ', '') + pay.[InvoiceNumber] 
FROM Payments AS pay 
WHERE pay.Order_id = @orderId 
ORDER BY pay.[InvoiceNumber] 
 
RETURN @InvoicesList

'

VistaDB presently returns the last column value.  Please vote this up!