SQL Lab Query Assistance - sql

Question

CNum DNum RNum Quant Price
C100 D1 R10 2 8.99
C100 D1 R40 7 9.99
C200 D3 R10 4 16.99
C200 D3 R20 2 15.99
C200 D3 R30 2 17.99
C200 D3 R40 5 19.99
C200 D3 R50 6 18.99
Hey guys, the above table is Named "Orders." I have a few queries I'm trying to solve for a Lab for a Database Management class I'm currently taking. A few of these queries are giving me some issues, if some of you wouldn't mind helping me out or walking me through some that would be great. The one I'm stuck on now is stated
"Get customer numbers (CNum) for customers that ordered dish D1 from some restaurant in a quantity greater than the average quantity of dish D1 for that restaurant."
another that I am having some trouble with is quite similar
"Get restaurant numbers (RNum) for restaurants that have orders of dish D1 in an average quantity less than the greatest quantity for any dish ordered from restaurant R10"
I gave #2 here a shot and came up with the following:
SELECT DISTINCT O.RNum
FROM Orders AS O
WHERE (O.DNum = 'D1')
AND EXISTS
(SELECT AVG(O.Quant) AS AvgQuant
FROM Orders AS O
WHERE EXISTS
(SELECT MAX(O.Quant) AS MaxQuant
FROM Orders AS O
WHERE (O.RNum = 'R10')))
With that in mind I currently believe this query to be incorrect due to the fact that I have not specified that the AvgQuant must be LESS THAN the MaxQuant, and am unsure of what the correct syntax would be. So my question here would be, how do I express that the AvgQuant must be less than the MaxQuant, and we have not used joins yet in this class.

Answer 1

For the first question:
Build an aggregate query that returns the average quantity for each restaurant and dish.
Build an aggregate query that returns the sum quantity for each customer and restaurant and dish.
Build a query that joins the first two on the restaurant and dish fields.
SELECT Query2.CNum, Query2.RNum, Query2.DNum, Query2.Cust_Rest_Dish_Sum
FROM Query2 INNER JOIN Query1 ON (Query2.DNum = Query1.DNum) AND (Query2.RNum = Query1.RNum)
WHERE (((Query2.Cust_Rest_Dish_Sum)>[Rest_Dish_Avg]));
Without JOIN clause:
SELECT Query2.CNum, Query2.RNum, Query2.DNum, Query2.Cust_Rest_Dish_Sum, Query1.RNum, Query1.DNum
FROM Query2, Query1
WHERE (((Query2.Cust_Rest_Dish_Sum)>[Rest_Dish_Avg]) AND ((Query1.RNum)=[Query2].[RNum]) AND ((Query1.DNum)=[Query2].[DNum]));
The data sample seems inadequate to test the query. No records will return. So I added another record and it is the only customer returned in the query.
CNum DNum RNum Quant Price
C200 D1 R10 5 22.49
Consider this for question 2 - no join used but really should:
SELECT DISTINCT Query4.RNum, Query4.DNum, Query4.AvgOfQuant, Query3.MaxOfQuant
FROM (SELECT Orders.RNum, Max(Orders.Quant) AS MaxOfQuant
FROM Orders
GROUP BY Orders.RNum
HAVING (((Orders.RNum)="R10"))) AS Query3, (SELECT Orders.RNum, Orders.DNum, Avg(Orders.Quant) AS AvgOfQuant
FROM Orders
GROUP BY Orders.RNum, Orders.DNum) AS Query4
WHERE (((Query4.DNum)="D1") AND ((Query4.AvgOfQuant)<[MaxOfQuant]));