相关子查询的MySQL范围界定问题 [英] MySql scoping problem with correlated subqueries

问题描述

我正在执行此Mysql查询，它的工作原理是:

I'm having this Mysql query, It works:

SELECT 
    nom
    ,prenom
    ,(SELECT GROUP_CONCAT(category_en) FROM
            (SELECT DISTINCT category_en FROM categories c WHERE id IN
                (SELECT DISTINCT category_id FROM m3allems_to_categories m2c WHERE m3allem_id = 37)
            ) cS
      ) categories
    ,(SELECT GROUP_CONCAT(area_en) FROM 
            (SELECT DISTINCT  area_en FROM areas c WHERE id IN 
                (SELECT DISTINCT area_id FROM m3allems_to_areas m2a WHERE m3allem_id = 37)
            ) aSq
     ) areas
FROM m3allems m
WHERE m.id = 37     

结果是:

nom             prenom      categories              areas
Man             Multi       Carpentry,Paint,Walls   Beirut,Baalbak,Saida

它工作正确，但仅当我将所需的ID硬编码到查询中时(37). 我希望它适用于m3allem表中的所有条目，所以我尝试这样做:

It works correclty, but only when i hardcode into the query the id that I want (37). I want it to work for all entries in the m3allem table, so I try this:

SELECT 
    nom
    ,prenom
    ,(SELECT GROUP_CONCAT(category_en) FROM
            (SELECT DISTINCT category_en FROM categories c WHERE id IN
                (SELECT DISTINCT category_id FROM m3allems_to_categories m2c WHERE m3allem_id = m.id)
            ) cS
      ) categories
    ,(SELECT GROUP_CONCAT(area_en) FROM 
            (SELECT DISTINCT  area_en FROM areas c WHERE id IN 
                (SELECT DISTINCT area_id FROM m3allems_to_areas m2a WHERE m3allem_id = m.id)
            ) aSq
     ) areas
FROM m3allems m

我得到一个错误:

"where"中的未知列"m.id" 条款"

Unknown column 'm.id' in 'where clause'

为什么? 从MySql手册中:

Why? From the MySql manual:

13.2.8.7. Correlated Subqueries 
[...] 
Scoping rule: MySQL evaluates from inside to outside.

那么...当子查询位于SELECT部分​​中时，这不起作用吗?我什么都没读.

So... do this not work when the subquery is in a SELECT section? I did not read anything about that.

有人知道吗?我该怎么办?我花了很长时间来构建此查询...我知道这是一个庞然大物的查询，但是它在单个查询中就可以满足我的要求，而且我已经接近使它开始工作了！

Does anyone know? What should I do? It took me a long time to build this query... I know it's a monster query but it gets what I want in a single query, and I am so close to getting it to work!

任何人都可以帮忙吗?

推荐答案

您只能将一级关联.

使用:

   SELECT m.nom,
          m.prenom,
          x.categories,
          y.areas
     FROM m3allens m
LEFT JOIN (SELECT m2c.m3allem_id,
                  GROUP_CONCAT(DISTINCT c.category_en) AS categories
             FROM CATEGORIES c
             JOIN m3allems_to_categories m2c ON m2c.category_id = c.id
         GROUP BY m2c.m3allem_id) x ON x.m3allem_id = m.id
LEFT JOIN (SELECT m2a.m3allem_id,
                  GROUP_CONCAT(DISTINCT a.area_en) AS areas
             FROM AREAS a
             JOIN m3allems_to_areas m2a ON m2a.area_id = a.id
         GROUP BY m2a.m3allem_id) y ON y.m3allem_id = m.id
    WHERE m.id = ?

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