I've got a query that is pulling some data. Here is my `WHERE` clause. 

```
where p.invc_dt  BETWEEN 1150101 AND 1160131    
and o.Tracking_num is not null 

```

This table contains order data, and an `ORDER_CD` column to identify what type of sale or if it was refund. Refund **does not** have a tracking number associated. How can I adjust my where statement to say something like 

```
and o.Tracking_num is not null **unless** `ORDER_CD` is REFUND

```