为什么 MySQL 告诉我该列不存在？答案

【问题标题】：Why does MySQL tell me that the column doesn't exist?为什么 MySQL 告诉我该列不存在？
【发布时间】：2013-11-21 10:57:53
【问题描述】：

我有 3 张桌子：关联（ID，名称）种族（身份证，姓名） stats(id,date,associationId,typeId,quantity)

我的目标是获得所有关联的列表，以及所有关联的类型列表，其中包含数量，首先是关联的总数，然后是每种类型。为此，我有这个查询：

SELECT association.name AS associationName,
        race.name AS raceName,
        SUM(report.quantity) AS quantity,
        subrequest.totalquantity as totalquantity
    FROM stats report
    JOIN association ON association.id = report.associationId
    JOIN race ON race.id = report.raceId
    JOIN (
        SELECT SUM(report.quantity) AS totalquantity,
            association.id AS subId
        FROM stats report
        JOIN association ON association.id = report.associationId
        WHERE date LIKE "2013-11-%"
        GROUP BY association.id
        ORDER BY totalquantity DESC
    ) subrequest ON subrequest.subId = association.id
    WHERE date LIKE "2013-11-%"
    GROUP BY association.id, race.id
        ORDER BY totalquantity DESC, quantity DESC

它工作得很好！但是现在，我只想为每个关联提供两个最大的类型。我试过这个：

set @num := 0, @association := '';
select associationName, raceName, quantity, totalquantity,
@num := if(@association = associationName, @num + 1, 1) as row_number,
        @association := associationName as dummy
FROM (
    SELECT association.name AS associationName,
        race.name AS raceName,
        SUM(report.quantity) AS quantity,
        subrequest.totalquantity as totalquantity
    FROM stats report
    JOIN association ON association.id = report.associationId
    JOIN race ON race.id = report.raceId
    JOIN (
        SELECT SUM(report.quantity) AS totalquantity,
            association.id AS subId
        FROM stats report
        JOIN association ON association.id = report.associationId
        WHERE date LIKE "2013-11-%"
        GROUP BY association.id
        ORDER BY totalquantity DESC
    ) subrequest ON subrequest.subId = association.id
    WHERE date LIKE "2013-11-%"
    GROUP BY association.id, race.id
    ORDER BY totalquantity DESC, quantity DESC
) x
WHERE row_number <= 2

有了这个，我有一个错误：#1054 - 'where 子句'中的未知列'row_number' 如果我删除 WHERE 子句，它工作正常，我得到了预期的结果。我也有一列 row_number 具有正确的数字。

我必须这样做才能使它工作：

set @num := 0, @association := '';
select associationName, raceName, quantity, totalquantity
FROM (
    select associationName, raceName, quantity, totalquantity,
    @num := if(@association = associationName, @num + 1, 1) as row_number,
            @association := associationName as dummy
    FROM (
        SELECT association.name AS associationName,
            race.name AS raceName,
            SUM(report.quantity) AS quantity,
            subrequest.totalquantity as totalquantity
        FROM association_race_report report
        JOIN association ON association.id = report.associationId
        JOIN race ON race.id = report.raceId
        JOIN (
            SELECT SUM(report.quantity) AS totalquantity,
                association.id AS subId
            FROM association_race_report report
            JOIN association ON association.id = report.associationId
            WHERE date LIKE "2013-11-%"
            GROUP BY association.id
            ORDER BY totalquantity DESC
        ) subrequest ON subrequest.subId = association.id
        WHERE date LIKE "2013-11-%"
        GROUP BY association.id, race.id
        ORDER BY totalquantity DESC, quantity DESC
    ) x
)y
WHERE row_number <= 2

这很好用！但是我不明白为什么我需要添加另一个级别的请求才能使 WHERE 子句起作用？添加一个无用的查询级别真的让我很恼火。有没有办法让它工作？为什么 MySQL 会引发此错误？

谢谢！

【问题讨论】：

我终于找到了解决办法！我只需要将 WHERE row_number

标签： mysql subquery

【解决方案1】：

MySQL 不喜欢列引用，因为它是在查询的选择部分中定义的——它还不能用于连接和 where 子句中。

解决方案，我看到你已经找到了，都是用定义它的表达式替换别名，或者创建一个中间子查询以避免重写整个事情。（查询优化器通常会删除中间查询。）

【讨论】：