MySQL子查询引用父查询中的字段

Question

我正在构建一个对评级数据执行一些过滤的查询。

假设我有一个名为 ratings 的简单表，如下所示，存储来自在线评级工具的数据:

+----------------+----------------+--------+
| page_title     | timestamp      | rating |
+----------------+----------------+--------+
| Abc            | 20110417092134 | 1      |
| Abc            | 20110418110831 | 2      |
| Def            | 20110417092205 | 3      |
+----------------+----------------+--------+

I need to extract pages with a high frequency of low values in the latest 10 ratings, and limit this query to pages that produced a volume of at least 20 ratings in the preceding week. This is the ridiculously long query I came up with:

SELECT a1.page_title, COUNT(*) AS rvol, AVG(a1.rating) AS theavg, 
(
     SELECT COUNT(*) FROM
     (
         SELECT * FROM ratings a2 WHERE a2.page_title = a1.page_title 
         AND DATE(timestamp) <= '2011-04-24' ORDER BY timestamp DESC LIMIT 10
     ) 
     AS latest WHERE rating >=1 AND rating <=2 ORDER BY timestamp DESC
)
AS lowest FROM ratings a1
WHERE DATE(a1.timestamp) <= "2011-04-24" AND DATE(a1.timestamp) >= "2011-04-17" 
GROUP BY a1.page_title HAVING COUNT(*) > 20

顶级查询查找在 2011-04-24 结束的一周内评分超过 20 的页面，子查询应该从最近的 10 个评分中检索值在 [1,2] 之间的评分数量顶级查询中的每篇文章。

MySQL提示子子查询的WHERE子句中的a1.page_title是一个未知列，我怀疑这是因为a1在二级查询中没有定义为别名，而只在顶级查询中，但是我不知道如何解决这个问题。

(已编辑)

我在上面添加了关于跨级别引用另一个绝对正常的查询的怀疑的解释，请注意，这里 a1 未在子查询中定义，但它在直接父查询中:

SELECT a1.page_title, COUNT(*) AS rvol, AVG(a1.rating) AS theavg, 
(
    SELECT COUNT(*) FROM ratings a2 WHERE DATE(timestamp) <= '2011-04-24'
    AND DATE(timestamp) >= '2011-04-17' AND rating >=1 
    AND rating <=2 AND a2.page_title = a1.page_title
) AS lowest FROM ratings a1 
WHERE DATE(a1.timestamp) <= '2011-04-17' AND DATE(a1.aa_timestamp) >= '2011-04-11' 
GROUP BY a1.page_title HAVING COUNT(*) > 20

Answer 1

我认为您可能会考虑加入两个在线 View ，这可能会使事情变得更容易。

SELECT * 
FROM   (SELECT COUNT(*), 
               a2.page_title 
        FROM   ratings a2 
        WHERE  DATE(timestamp) <= '2011-04-24' 
               AND DATE(timestamp) >= '2011-04-17' 
               AND rating >= 1 
               AND rating <= 2 

        GROUP  BY a2.page_title) current 
       JOIN 
        (SELECT a1.page_title, 
                    COUNT(*)       AS rvol, 
                    AVG(a1.rating) AS theavg 
             FROM   ratings a1 
             WHERE  DATE(a1.timestamp) <= '2011-04-17' 
                    AND DATE(a1.a_timestamp) >= '2011-04-11' 
             GROUP  BY a1.page_title 
             HAVING COUNT(*) > 20) morethan20 
         ON current .page_title = morethan20.page_title