SELECT co.custid, co.name, iif(isNull(sa.price),"",sa.price), ...

or something like this is what I use to turn NULL values into an empty string. You could use ... ,0,sa.price) ... if you want the number zero to be returned in place of a NULL value.

It sounds like you are wanting to return all records in your customer table, so you don't need a WHERE clause.

Is this what you are looking for?