仅当 id 不同时才在 group_concat 中获取重复值

Question

我有两个表，首先是表 Product：

id|category_id
--+-----------
1 | 12345
2 | 12345
3 | 12465

然后是表活动：

id|prod_id|activity_type   |description
--+-------+----------------+-----------
1 | 1     | Initialization | blah
2 | 1     | Finalization   | foo
3 | 2     | Initialization | blah again
4 | 2     | Duplication    | bar
5 | 2     | Finalization   | foobar
6 | 3     | Initialization | blob
7 | 3     | Migration      | A to B
8 | 3     | Migration      | B to C
9 | 3     | Finalization   | fuh

现在我想为每种类型的活动检索至少具有一种此类活动的产品数量，以及产品类别列表。类别将在此类别的每个产品的列表中重复。现在我正在使用以下查询：

SELECT a.activity_type as Activity, COUNT(DISTINCT p.id) as Products,
CONVERT(GROUP_CONCAT(p.category SEPARATOR ',  ') USING utf8) AS Categories
FROM mydb.product p, mydb.activity a
WHERE p.id = a.prod_id
AND a.activity_type <> '' // To not count activities which haven't been correctly initialized
GROUP BY Categories
ORDER BY Products

现在我等待的结果是：

Activity       | Products | Categories
---------------+----------+--------------------
Initialization | 3        | 12345, 12345, 12465
Finalization   | 3        | 12345, 12345, 12465
Duplication    | 1        | 12345
Migration      | 1        | 12465

但是通过这个查询，我得到了'12465, 12465'迁移的价值。我可以得到一个类别出现在列表中，仅针对每个不同的产品 ID，而不是针对一种类型的每个活动？

Answer 1

双方第一组activity_type和prod_id，然后另一组由activity_type：

SELECT 
    a.activity_type AS Activity, 
    COUNT(DISTINCT p.id) AS Products,
    CONVERT(GROUP_CONCAT(p.category_id SEPARATOR ',  ') USING utf8) 
      AS Categories
FROM 
    product AS p
  JOIN 
    ( SELECT activity_type
           , prod_id
      FROM activity 
      WHERE activity_type <> '' 
      GROUP BY activity_type
             , prod_id
    ) AS a
    ON p.id = a.prod_id
GROUP BY 
    activity_type
ORDER BY 
    Products DESC;

在SQL-Fiddle 中测试（谢谢@Mr.Radical）

您也可以安全地替换上面的COUNT(DISTINCT p.id)with COUNT(*)，对于每个活动类型，只有不同的产品 ID（这在内部 group by 中进行了处理）。