SQL - improve NOT EXISTS query performance

Question

Is there a way I can improve this kind of SQL query performance:

INSERT
INTO ...
WHERE NOT EXISTS(Validation...)

The problem is when I have ma

Answer 1

insert into customers 
select * 
from newcustomers 
where customerid not in (select customerid 
                         from customers)

..may be more efficient. As others have said, make sure you've got indexes on any lookup fields.