mysql - 计算外键数

Question

我有三张桌子。

1.fi_category

+----+-----------------+-----------------+
| id | name            | slug            |
+----+-----------------+-----------------+

2.fi_subcategory

+----+-----------------+-----------------+-------------+
| id | name            | slug            | category_id |
+----+-----------------+-----------------+-------------+

3.fi_business_subcategory

+----+-------------+----------------+
| id | business_id | subcategory_id |
+----+-------------+----------------+

我基本上想做的是，

1. 获取所有类别
2. 获取属于类别的所有子类别。
3. 计算特定子类别存在的业务数量。

这就是我尝试做的。

SELECT 
    f.id,
    f.name,
    f.slug,
    f2.id,
    f2.name,
    f2.slug,
    COUNT(f3.business_id) as count
FROM
    fi_category f
LEFT JOIN 
    fi_subcategory f2 ON f.id = f2.category_id
LEFT JOIN 
    fi_business_subcategory f3 ON f2.id = f3.subcategory_id

但是上面的查询只获取 1 条记录。我如何获取我想要的东西？

Answer 1

我会添加一个GROUP BY子句：

SELECT 
    f.id,
    f.name,
    f.slug,
    f2.id,
    f2.name,
    f2.slug,
    COUNT(f3.business_id) as count
FROM fi_category f
LEFT JOIN fi_subcategory f2 
    ON f.id = f2.category_id
LEFT JOIN fi_business_subcategory f3 
    ON f2.id = f3.subcategory_id
GROUP BY f.id,
    f.name,
    f.slug,
    f2.id,
    f2.name,
    f2.slug

或者让你count()的子查询：

SELECT f.id,
    f.name,
    f.slug,
    f2.id,
    f2.name,
    f2.slug,
    f3.cnt
FROM fi_category f
LEFT JOIN fi_subcategory f2 
    ON f.id = f2.category_id
LEFT JOIN
(
    select count(business_id) cnt, subcategory_id 
    from fi_business_subcategory
    group by subcategory_id
) f3 
    ON f2.id = f3.subcategory_id