Sql Devloper - Use concat with GROUP BY function for COUNTING (Error ORA-00904) : I am working on a mock database for a project and
I have two tables: CUSTOMERS AND ORDERS
Both tables look like this

CUSTOMERS: custID, custLName, custFName, custAddress, custTown, custPostcode, custPhone, custEmail

And

ORDERS: orderID, orderDate, dispatchDate, custID (foreign key)

I am trying to generate a query that returns the full name and phone number of the customer who has made the most orders. 
This is my query below however it is returning an error on the GROUP BY function stating ORA-00904: "CUSTOMER": invalid identifier

    SELECT b.custFName || ' ' || b.custLName || ', ' || b.custPhone AS Customer, 
    COUNT(DISTINCT o.custID) AS Orders_Placed
    FROM CUSTOMERS b, ORDERS o 
    GROUP BY Customer
    HAVING COUNT(DISTINCT o.custID) AND b.custID = o.custID
    ORDER BY o.custID DESC;


