How to update the same field twice in mysql database

Question

I have a question regarding UPDATE in mysql.

I have a customer table linked to a customer_orders table. The customer table has a customer_balance field. The customer_orders table has an order_cost field . I want to run a monthly query to add the amounts from the order_cost field to the customer_balance field.

I have tried to use UPDATE and SET but this only works if the customer has one order. Any additional order_cost amounts are not added.

Any ideas on how to do this greatly appreciated.

Answer 1

You want to aggregate before doing the update . Something like this:

update customer c join
       (select customerid, sum(order_cost) as sumoc
        from customer_orders co
        group by customerid
       ) co
       on c.customerid = co.customerid
    set c.customer_balance = c.customber_balance + co.sumoc;

Answer 2

Why taking such a risk with data consistency

select customerid, sum(order_cost) as sumoc
    from customer_orders co
    group by customerid

This will give you always real-time en consistent data.

If you're having performance issues, consider a (after insert) trigger on the custumer_orders table wich updates the customer balance every time an order is inserted