Hi Krishna,
Above behavior is expected. If you would like to get customer details from both the tables, please try like this.
Select T2."ORDERID",T1."CUSTOMERID",T2."CUSTOMERID",T1."CUSTOMERNAME",T2."PRODUCT",T2."TOTAL_UNITS" from "CUSTOMER" AS T1 FULL OUTER JOIN "ORDERS"AS T2 ON T1."CUSTOMERID" = T2."CUSTOMERID".
Regards,
sai krishna.