Question

我有一个listing_categories表，其中包含许多listing_plans和Listings，以及许多子类别。我试图编写一个视图以显示带有Listing_plan计数，清单计数和子类别计数的清单类别。

这是我的listing_categories表：

MariaDB [railsapp_development]> describe listing_categories;
+---------------------+--------------+------+-----+---------+----------------+
| Field               | Type         | Null | Key | Default | Extra          |
+---------------------+--------------+------+-----+---------+----------------+
| id                  | bigint(20)   | NO   | PRI | NULL    | auto_increment |
| website_id          | int(11)      | YES  | MUL | NULL    |                |
| listing_category_id | int(11)      | YES  |     | NULL    |                |
| name                | varchar(255) | YES  |     | NULL    |                |
| slug                | varchar(255) | YES  |     | NULL    |                |
| description         | text         | YES  |     | NULL    |                |
| category_card       | text         | YES  |     | NULL    |                |
| listing_card        | text         | YES  |     | NULL    |                |
| layout              | varchar(255) | YES  |     | NULL    |                |
| status              | int(11)      | YES  |     | NULL    |                |
| created_at          | datetime     | NO   |     | NULL    |                |
| updated_at          | datetime     | NO   |     | NULL    |                |
| keywords            | varchar(255) | YES  |     | NULL    |                |
+---------------------+--------------+------+-----+---------+----------------+

还有我要写的视图：

create or replace view `listing_category_details` AS
    select
        cats1.*,
        count(l.id) as listing_count,
        count(cats2.id) as subcategory_count,
        count(lp.id) as plan_count
    from
        listing_categories as cats1
    left join
        listings as l on cats1.id = l.listing_category_id
    left join
        listing_plans as lp on cats1.id = lp.listing_category_id
    inner join
        listing_categories as cats2 on cats1.listing_category_id = cats2.id
    group by
        cats1.id
    order by
        cats1.name asc;

输出不正确，它没有显示正确的计数，如下所示：

MariaDB [railsapp_development]> select id, slug, name, listing_count, subcategory_count, plan_count from listing_category_details limit 10;
+----+-------------------+-------------------+---------------+-------------------+------------+
| id | slug              | name              | listing_count | subcategory_count | plan_count |
+----+-------------------+-------------------+---------------+-------------------+------------+
| 17 | ares              | Ares              |            22 |                22 |         22 |
| 30 | automotive        | Automotive        |            16 |                16 |         16 |
| 19 | crist-osinski-inc | Crist-Osinski Inc |            12 |                12 |         12 |
| 29 | esl-cologne       | ESL Cologne       |            20 |                20 |         20 |
| 18 | executive-office  | Executive Office  |            22 |                22 |         22 |
| 27 | gfinity-london    | GFinity London    |            24 |                24 |         24 |
| 25 | hephaestus        | Hephaestus        |            28 |                28 |         28 |
| 24 | iem-championship  | IEM Championship  |            14 |                14 |         14 |
| 26 | league-all-stars  | League All Stars  |            30 |                30 |         30 |
| 21 | machinery         | Machinery         |            14 |                14 |         14 |
+----+-------------------+-------------------+---------------+-------------------+------------+

在上面的示例中，没有一个数字是必填项，例如，每个listing_category都有2个计划。

我在做什么错了？

Answer 1

使用COUNT(DISTINCT)：

select cats1.*,
       count(distinct l.id) as listing_count,
       count(distinct cats2.id) as subcategory_count,
       count(distinct lp.id) as plan_count
from listing_categories  cats1 left join
     listings l
     on cats1.id = l.listing_category_id left join
     listing_plans lp
     on cats1.id = lp.listing_category_id inner join
     listing_categories cats2
     on cats1.listing_category_id = cats2.id
group by cats1.id
order by cats1.name asc;

COUNT()仅计算非NULL值的数量。您正在沿着层次结构进行联接，因此当您下降层次结构时，行数将成倍增加。

通过嵌套类别表中的计数进行自我连接

1 个答案: