多个as 显示缺少关键字，为啥啊？

[Fer12138]

SELECT s.STUDENT_ID , s.COURSE_ID ,s.SCORE_NUMBER ,a_s_n
FROM
	TRAIN_SCORE s
JOIN 
	(SELECT
		a.STUDENT_ID ,
		AVG(a.SCORE_NUMBER) AS a_s_n
	FROM
		TRAIN_SCORE a
	GROUP BY
		STUDENT_ID ) AS b
ON
	s.STUDENT_ID=b.STUDENT_ID
ORDER BY 
	b.a_s_n  DESC

下面是正确的

--13. 按平均成绩从高到低显示所有学生的所有课程的成绩以及平均成绩
SELECT s.STUDENT_ID , s.COURSE_ID ,s.SCORE_NUMBER ,a_s_n
FROM
	TRAIN_SCORE s
JOIN 
	(SELECT
		a.STUDENT_ID ,
		AVG(a.SCORE_NUMBER) AS a_s_n
	FROM
		TRAIN_SCORE a
	GROUP BY
		STUDENT_ID ) b
ON
	s.STUDENT_ID=b.STUDENT_ID
ORDER BY 
	b.a_s_n  DESC

就在b表前少了个as。有大佬能说明一下吗？

[weixin_45534996]

@weixin_45534996 连接不是链接

[weixin_45534996]

缺少的应该是ON ，AS被当做表名，之后应该链接ON，而你后面是b