A PL/SQL example of a for loop that prints out the primary key for all rows returned by a query:

DECLARE
  ID VARCHAR2(50);
BEGIN
    DBMS_OUTPUT.ENABLE (1000000);
    FOR current_row IN (SELECT id FROM your_table)
    LOOP
        ID := current_row.id;    -- Assign value to variable
        DBMS_OUTPUT.PUT_LINE( '' || ID);
        DBMS_OUTPUT.PUT_LINE( '' || current_row.column_xxx);
    END LOOP;
END;
/