MySql scoping problem with correlated subqueries

问题

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

The result is:

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

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

And I get an error:

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

Why? From the MySql manual:

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

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

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!

Can anyone help?

回答1:

You can only correlate one level deep.

Use:

   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 = ?

回答2:

The reason for the error is that in the subquery m is not defined. It is defined later in the outer query.

来源：https://stackoverflow.com/questions/2435644/mysql-scoping-problem-with-correlated-subqueries

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