It's a simple question, I'm getting started with SQL and I'm having a lot of problems with it. 

I have TBL\_SALES and TBL\_CUSTOMERS. Both can be JOINed by the field CUSTOMER\_ID. 
A customer may have a CUSTOMER\_ID and have no entry on TBL\_CUSTOMERS

I would like to FITER all entries on TBL\_SALES that don't have an entry on TBL\_CUSTOMERS.

Which I think it's the opposite of 

```
SELECT UNIQUE TBL_SALES.CUSTOMER_ID, TBL_CUSTOMERS.CUSTOMER_ID
FROM TBL_SALES, TBL_CUSTOMERS
WHERE TBL_SALES.CUSTOMER_ID = TBL_CUSTOMERS.CUSTOMER_ID

```

Well... I can do that with a VBA code using SEEK and NOMATCH. But I know it's not productive. And if I try to use this code, it doesn't work:

```
SELECT UNIQUE TBL_SALES.CUSTOMER_ID, TBL_CUSTOMERS.CUSTOMER_ID
FROM TBL_SALES, TBL_CUSTOMERS
WHERE TBL_SALES.CUSTOMER_ID <> TBL_CUSTOMERS.CUSTOMER_ID

```

I realized that there must be a 'SEEK' command in SQL but I'm pretty sure that is a smarter way to run this query.