Unknown column in 'field list' IN SQL

 DELIMITER //
 CREATE FUNCTION fnc_credit_custstatus
 RETURNS VARCHAR(6) DETERMINISTIC
 BEGIN
 DECLARE custstatus VARCHAR(6);
 IF CustCredit>=1000 THEN SET custstatus='VIP';
 ELSEIF CustCredit<1000 THEN SET custstatus='NONVIP';
 END IF;
 RETURN custstatus;
 END //
 DELIMITER ;

I think I created FUNCTION has succeeded，but when I SELECT the function is Error Code: 1054 Unknown column 'fnc_credit_custstatus' in 'field list'

 SELECT*, fnc_credit_custstatus AS custstatus
 FROM customer_salon

You do have to pass a parameter..and this doesn't syntax because you don't define one.

drop function if exists f;

DELIMITER //
 CREATE FUNCTION f(custcredit int)
 RETURNS VARCHAR(6) DETERMINISTIC
 BEGIN
 DECLARE custstatus VARCHAR(6);
 IF CustCredit>=1000 THEN SET custstatus='VIP';
 ELSEIF CustCredit<1000 THEN SET custstatus='NONVIP';
 END IF;
 RETURN custstatus;
 END //
 DELIMITER ;
 set @cr = 2000;
 select @cr,f(@cr);

+------+--------+
| @cr  | f(@cr) |
+------+--------+
| 2000 | VIP    |
+------+--------+
1 row in set (0.001 sec)

or in your case

SELECT *, f(custcredit) AS custstatus FROM customer_salon