与UNION一起使用ORDER BY：NULL问题

Question

我为所有内容提供了正确的数字，但是当我尝试使用ORDER BY子句时，它可以工作，但平均GPA行排在数据的顶部。应该在最后。

我尝试了ISNULL，GROUPING，但它们都没有起作用。

SELECT se.sectionid AS sectionid, st.lname AS last_name, st.fname AS first_name, g.letter AS letter, FORMAT(AVG(gs.point),2) AS gpa 
FROM student as st, section as se, grade as g, grade_scale as gs 
WHERE st.studentid = g.studentid AND se.sectionid = g.sectionid AND g.letter = gs.letter AND se.sectionid = 35
GROUP BY sectionid, last_name, first_name, letter
UNION
SELECT 'Average GPA','', '','', FORMAT(AVG(gs.point),2) AS gpa 
FROM student as st, section as se, grade as g, grade_scale as gs 
WHERE st.studentid = g.studentid AND se.sectionid = g.sectionid AND g.letter = gs.letter AND se.sectionid = 35
ORDER BY last_name, first_name;

“平均GPA”行应位于底部，我在Google上搜索的任何内容均无效。该表应按last_name和first_name排序。

谢谢！