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。

我想执行查询以显示所有类别，该类别的活动项目总费用

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，并使用所有项而不管活动/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

结果如此：

+--------+------------+---------------+ | 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se，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......任何需要一百万个想象的酷点？

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

Answer 2

试试这个：

  SELECT a.*,  
    SUM(Case B.Active When 1 Then b.cost else 0 End) AS total_cost 
    FROM categories a 
       LEFT JOIN items b 
         ON b.category_id = a.id  
    GROUP BY a.category_name

或者这个：

    SELECT a.*, SUM(b.cost) AS total_cost 
    FROM categories a 
       LEFT JOIN items b 
         ON b.category_id = a.id 
            And B.Active = 1 
    GROUP BY a.category_name

mysql与SUM和WHERE子句左连接问题

2 个答案: