In real applications, we do not delete the records to deduct the points from the customers. Because every record with the orders are required for reporting and management purposes. Instead the best way to deduct 100 points from the customer is to add the points with a negative sign and then sum up all the points so that you'll get the final value.
For example do the following:
Insert new record with negative value.
insert into reward_points (customer_id,order_id,points) values (1,8,-100);
Then sum the total value as below.
select sum(points) as total_rewards where customer_id=1;
This way you'll keep all the records of the previous orders as well as reduce the rewards too.
Hope this helps for your requirement.