Group function is nested too deeply SQL Error - sql

Question

I have a table which looks like this:
+-----------------+--------------+
| Field | Type |
+-----------------+--------------+
| orderNumber (PK)| int |
| orderDate | date |
| requiredDate | date |
| shippedDate | date |
| status | char(15) |
| comments | char(200) |
| customerNumber | int |
+-----------------+--------------+
I need to return the customerNumber which has maximum number of orders.
I tried the following command:
SELECT customerNumber FROM ORDERS WHERE customerNumber IN (SELECT customerNumber FROM ORDERS HAVING MAX(COUNT(customerNumber)) GROUP BY customerNumber);
I think an error: group function is nested too deeply

Answer 1

SQL Fiddle
Oracle 11g R2 Schema Setup:
CREATE TABLE ORDERS (
orderNumber int PRIMARY KEY,
orderDate date,
requiredDate date,
shippedDate date,
status char(15),
comments char(200),
customerNumber int
);
INSERT INTO ORDERS ( ORDERNUMBER, CUSTOMERNUMBER ) VALUES ( 1, 1 );
INSERT INTO ORDERS ( ORDERNUMBER, CUSTOMERNUMBER ) VALUES ( 2, 1 );
INSERT INTO ORDERS ( ORDERNUMBER, CUSTOMERNUMBER ) VALUES ( 3, 2 );
INSERT INTO ORDERS ( ORDERNUMBER, CUSTOMERNUMBER ) VALUES ( 4, 2 );
INSERT INTO ORDERS ( ORDERNUMBER, CUSTOMERNUMBER ) VALUES ( 5, 3 );
INSERT INTO ORDERS ( ORDERNUMBER, CUSTOMERNUMBER ) VALUES ( 6, 4 );
Query 1 - If you only want to get a single customer:
SELECT CUSTOMERNUMBER
FROM (
SELECT CUSTOMERNUMBER,
COUNT( ORDERNUMBER ) AS num_orders
FROM ORDERS
GROUP BY CUSTOMERNUMBER
ORDER BY num_orders DESC
)
WHERE ROWNUM = 1
Results:
| CUSTOMERNUMBER |
|----------------|
| 1 |
Query 2 - If you want to get all customers with the highest number of orders:
SELECT CUSTOMERNUMBER
FROM (
SELECT CUSTOMERNUMBER,
RANK() OVER ( ORDER BY NUM_ORDERS DESC ) AS RNK
FROM (
SELECT CUSTOMERNUMBER,
COUNT( ORDERNUMBER ) AS num_orders
FROM ORDERS
GROUP BY CUSTOMERNUMBER
ORDER BY num_orders DESC
)
)
WHERE RNK = 1
Results:
| CUSTOMERNUMBER |
|----------------|
| 1 |
| 2 |

Answer 2

One way to do it is using ctes, where you get the count of orders in the first cte, then select the maximum value. Finally join them to get the customer with the maximum orders.
with ordercount as (select customernumber, count(distinct ordernumber) ordercount
from orders
group by customernumber)
,maxorders as (select max(ordercount) maxcount from ordercount)
select o.customernumber
from ordercount o
join maxorders m on m.maxcount = o.ordercount