MySQL ROLL Up question
I'm not sure what it is you are trying to do, the GROUP BY clause that you provided is invalid. Is the following what you meant?

SELECT coalesce (OrderID, 'GrandTotal') AS OrderID, 
       SUM(OrderPrice) as SumOrderPrice
FROM ORDERS 
WHERE OrderDate BETWEEN '2018-04-01' AND '2018-04-30'
GROUP BY OrderID WITH ROLLUP;


I.e. GROUP BY OrderID instead of OrderPrice?