为什么我可以对内联SELECT值进行排序，但不在WHERE子句中使用它？

I have this small SQL query.

我有这个小SQL查询。

SELECT a.`id` , a.`title` , a.`date` , 
(
    SELECT MAX( grade )
    FROM tests
    WHERE userid = 41
    AND presid = a.`id`
) AS grade
FROM `presentations` a
WHERE a.`visible` = 1
AND `grade` >= 5
ORDER BY `grade` DESC
SE