SQL query

Im trying to do a sql query whereby it will count the cust_id. The checking now is it will check the cust_id from 2 tables. customer and answer tbl whether the id appear in both table and whether survey_taken ='Y'. But if the cust_id did not appear in the customer tbl it will still return 0. I want to ensure that the customer appear in the customer tbl be4 he is allow to take the survey. Is there any other way I can do the query?

select count(O.cust_id) as cnt from customer C, answer O
where C.cust_id= O.cust_id and O.survey_taken='Y' and O.cust_id=8