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？

Question

我有这个小的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

这给了我错误

1054 - 'where子句'中的未知列'等级'

但如果我删除第二行,它工作正常.我试图做AND a.grade,甚至给测试表一个名字,并将该名称附加到等级,但仍然没有运气.

如何在WHERE子句中使用此内联查询？

我发现这有效,但这是唯一的方法吗？

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 (
    SELECT MAX( grade )
    FROM tests
    WHERE userid = 41
    AND presid = a.`id`
) >= 5
ORDER BY `grade` DESC

Answer 1

按以下顺序对Sql语句进行了某种程度的评估:

• 从
• 哪里
• 选择
• 组
• HAVING
• 订购

因此,您在SELECT子句中定义的内容在WHERE子句中不可用.您需要将该约束放入HAVING子句中:

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
HAVING `grade` >= 5
ORDER BY `grade` DESC