update table using another table
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i have to update total_orders of customers column to be equal to the total number of all the orders placed by the customer(in cust_order)![enter image description here][1]


  [1]: http://i.stack.imgur.com/gVJLu.png

here is what i have tried


    update (select * from atish_customer a inner join (select cust_nbr,count(cust_nbr) as count_orders
    from atish_cust_order group by cust_nbr)c
    on c.cust_nbr=a.cust_nbr) set tot_orders=count_orders;

But this is the error i get

    ORA-01779: cannot modify a column which maps to a non key-preserved table


