[英]mysql left join problem with SUM and WHERE clause
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相关的类别ID。
我想执行查询以显示所有类别,以及该类别活动项目的总成本
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 |
+--------+------------+---------------+
我的第一个查询:
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
可以正常工作,但是它返回NULL项而不是0,并且使用所有项,无论活动/不活动:
+--------+------------+---------------+
| id | cat_name | total_cost |
+--------+------------+---------------+
| 1 | cat 1 | 44 |
| 2 | cat 2 | NULL |
| 3 | cat 3 | 87 |
+--------+------------+---------------+
我的第二个查询处理了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
结果像这样:
+--------+------------+---------------+
| id | cat_name | total_cost |
+--------+------------+---------------+
| 1 | cat 1 | 44 |
| 2 | cat 2 | NULL |
| 3 | cat 3 | 87 |
+--------+------------+---------------+
因此,在我无用的大脑中,我尝试以下查询,在表b上添加WHERE子句,其中active必须为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 |
+--------+------------+---------------+
因此,正如您所能看到的,即使右表未返回匹配结果,我也想返回整个类别范围...一百万个虚构的酷点需要花费什么?
采用:
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
尝试这个:
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
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