mysql与SUM和WHERE子句左连接问题

Question

我的数据库中有2个表:项目和类别.项目可以是活动的,也可以是非活动的,并且具有与类别表中记录的id相关的categoryID.

我想执行一个查询,以显示所有类别,与活跃的项目总成本的类别

所以我的目标是返回看起来像这样的东西:

    +--------+------------+---------------+
    | id     | cat_name   | total_cost    |
    +--------+------------+---------------+
    |      1 | cat 1      | 12            |
    |      2 | cat 2      | 0             |
    |      3 | cat 3      | 45            |
    +--------+------------+---------------+

我的第一个问题:

    SELECT a.*, 
    SUM(b.cost) AS total_cost
    FROM categories a LEFT JOIN items b
    ON(a.id = b.category_id)
    GROUP BY a.category_name

工作正常,但它返回NULL项而不是0,并使用所有项目,无论活动/非活动:

    +--------+------------+---------------+
    | id     | cat_name   | total_cost    |
    +--------+------------+---------------+
    |      1 | cat 1      | 44            |
    |      2 | cat 2      | NULL          |
    |      3 | cat 3      | 87            |
    +--------+------------+---------------+

我的第二个查询解决了NULL值:

    SELECT a.*, 
    SUM(IF(b.cost IS NULL, 0, b.cost)) AS total_cost
    FROM categories a LEFT JOIN items b
    ON(a.id = b.category_id)
    GROUP BY a.category_name

结果是这样的:

    +--------+------------+---------------+
    | id     | cat_name   | total_cost    |
    +--------+------------+---------------+
    |      1 | cat 1      | 44            |
    |      2 | cat 2      | NULL          |
    |      3 | cat 3      | 87            |
    +--------+------------+---------------+

所以在我无用的大脑中,我尝试以下查询,在表b上添加一个WHERE子句,其中active为= 1(true)

    SELECT a.*, 
    SUM(IF(b.cost IS NULL, 0, b.cost)) AS total_cost
    FROM categories a LEFT JOIN items b
    ON(a.id = b.category_id)
    WHERE b.active = 1 
    GROUP BY a.category_name

我得到以下内容:

    +--------+------------+---------------+
    | id     | cat_name   | total_cost    |
    +--------+------------+---------------+
    |      1 | cat 1      | 12            |
    |      3 | cat 3      | 45            |
    +--------+------------+---------------+

所以你可以,我想返回整个范围的类别,即使正确的表没有返回匹配的结果...任何需要一百万个虚构的酷点？

Answer 1

使用:

   SELECT c.id,
          c.cat_name,
          COALESCE(SUM(i.cost), 0) AS total_cost
     FROM CATEGORIES c
LEFT JOIN ITEMS i ON i.category_id = c.category_id
                 AND i.active = 1
 GROUP BY c.id, c.cat_name