mysql - 为什么我可以对内联 SELECT 值进行排序但不能在 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'grade'

但是，如果我删除最后第二行，它工作正常。我已经尝试过AND a.grade，甚至给测试表命名并将该名称附加到成绩但仍然没有运气。

如何在 WHERE 子句中使用此内联查询？

我发现这行得通，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

Sql 语句在某种程度上按以下顺序进行评估：

• 从
• 在哪里
• 选择
• 团体
• 拥有
• 命令

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

Answer 2

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