给出一个客户订单表,列 OrderID,客户ID,订单日期和订单金额,写一个更新声明,设置订单金额为0所有订单,被放置超过6个月前,订单金额低于100美元.
Here is the UPDATE statement you can use:

```
UPDATE customer_orders
SET OrderAmount = 0
WHERE OrderDate <= DATEADD(month, -6, GETDATE()) AND OrderAmount < 100;
```

This statement will update the `OrderAmount` column to 0 for all customer orders that were placed more than 6 months ago and had an `OrderAmount` of less than $100. The `DATEADD` function is used to subtract 6 months from the current date, and the `WHERE` clause filters the results to only include orders that match the conditions.