使用 IN 和 group_concat 结果的 Mysql 查询

Question

我正在尝试清理包含重复记录的数据库。我需要将引用移动到一条记录并删除另一条记录。

我有两个表:Promoters 和 Venues，每个表都引用了一个名为 cities 的表。问题是有些城市同名不同id，与 field 和发起人有关系。

通过这个查询，我可以将所有发起人和场所与一个城市记录分组:

SELECT c.id as id, c.name as name, GROUP_CONCAT( DISTINCT p.id ) as promoters_ids, GROUP_CONCAT( DISTINCT v.id ) as venues_ids
FROM cities as c
LEFT JOIN promoters as p ON p.city_id = c.id
LEFT JOIN venues as v ON v.city_id = c.id
WHERE c.name IN ( SELECT name from cities group by name having count(cities.name) > 1 )
GROUP BY c.name

现在我想对发起人运行更新查询，将 city_id 设置为上面查询的结果。

像这样:

    UPDATE promoters AS pr SET pr.city_id = (
        SELECT ID
        FROM (
            SELECT c.id as id, c.name as name, GROUP_CONCAT( DISTINCT p.id ) as promoters_ids
            FROM cities as c
            LEFT JOIN promoters as p ON p.city_id = c.id

            WHERE c.name IN ( SELECT name from cities group by name having count(cities.name) > 1 ) AND pr.id IN promoters_ids
            GROUP BY c.name
            ) AS T1 

    )

我该怎么做？

谢谢

Answer 1

如果我没理解错的话，您想要删除重复的城市(最后)，因此您需要更新链接到您要在此过程中删除的任何城市的推广者。

我认为使用任何具有相同名称的城市的最低 ID 是有意义的(也可以是最高的 ID，但我至少想指定它，不要让我来决定。

因此，为了获得发起人的正确 ID，我需要:选择与已链接到发起人的城市同名的所有城市的最低 ID。

幸运的是，这种需求恰好适合查询:

UPDATE promoters AS pr 
SET pr.city_id = (
  SELECT 
    -- Select the lowest ID ..
    Min(c.id)
  FROM
    -- .. of all cities ..
    Cities c
    -- .. that have the same name ..
    INNER JOIN Cities pc on pc.Name = c.Name
  WHERE
    .. as the city already linked to the promoter being updated
    pc.id = pr.city_id
  GROUP BY
    c.name)

诀窍是按名称加入 Cities 本身，因此您可以轻松获得所有同名的城市。我认为您对 IN 子句进行了同样的尝试，但这比需要的要复杂一些。

我认为您根本不需要 group_concat，除了检查内部查询是否确实返回了正确的城市，尽管这没有意义，因为您已经根据名称进行分组.当这样写时，您可以知道这应该不会出错:

  SELECT 
    -- Select the lowest ID ..
    MIN(c.id) AS id,
    GROUP_CONCAT(c.name) AS names --< already grouped by this, so why...
  FROM
    -- .. of all cities ..
    Cities c
    -- .. that have the same name.
    INNER JOIN Cities pc on pc.Name = c.Name
  GROUP BY
    c.name

我希望我正确理解了这个问题。