爲什麼我可以對內聯SELECT值進行排序，但不能在WHERE子句中使用它？

Question

我有這個小的SQL查詢。

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 

這給我的錯誤

1054 - 未知列 '品位' 在 'where子句'

但如果我刪除第二個最後一行，它工作正常。我試圖做AND a.grade，甚至給測試表一個名字，並追加這個名字來評分，但仍然沒有運氣。

如何在WHERE子句中使用此內嵌查詢？

我發現這是有效的，但它是唯一的方法嗎？

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語句是按照下面的順序評價稍微：

• FROM
• WHERE
• SELECT
• GROUP
• HAVING
• ö RDER

因此，您在SELECT子句中定義的內容在WHERE子句中不可用。您需要將該約束放入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