Question

我需要选择所有类别及其子类别的类别。

这里假设我的表：

类别

id | title
----------
1  | colors
2  | animals
3  | plants

- 行业标准

id | category_id | title | confirmed
------------------------------------
1        1          red       1
2        1          blue      1
3        1          pink      1
4        2          cat       1
5        2          tiger     0
6        2          lion      0

我想要的是：

id |  title  | count
------------------
1    colors     3
2    animals    1
3    plants     0

到目前为止我尝试过：

SELECT c.id, c.title, count(s.category_id) as count from categories c
LEFT JOIN sub_categories s on c.id = s.category_id
WHERE c.confirmed = 't' AND s.confirmed='t'
GROUP BY c.id, c.title
ORDER BY count DESC

此查询的唯一问题是此查询不显示具有0个子类别的类别！

您也可以在SqlFiddle

上查看

任何帮助都会非常感激。

Answer 1

您没有得到零计数行的原因是WHERE子句将s.confirmed检查为t，从而消除了外连接中带有NULL s的行结果

将s.confirmed检查移入连接表达式以解决此问题：

SELECT c.id, c.title, count(s.category_id) as count from categories c
LEFT JOIN sub_categories s on c.id = s.category_id AND s.confirmed='t'
WHERE c.confirmed = 't' 
GROUP BY c.id, c.title
ORDER BY count DESC

添加Sql小提琴：http://sqlfiddle.com/#!17/83add/13

Answer 2

我认为你也可以试试这个（它证明了你真正分组的列）：

SELECT c.id, c.title, RC
from categories c
LEFT JOIN (SELECT category_id, COUNT(*) AS RC 
           FROM sub_categories 
           WHERE confirmed= 't' 
           GROUP BY category_id) s on c.id = s.category_id
WHERE c.confirmed = 't' 
ORDER BY RC DESC

选择包含COUNT个子类别

2 个答案: