SQL max函数

Question

我有以下查询：

SELECT q.category_id as Category_id , COUNT(q.question_id) as count 
from questions as q 
    INNER JOIN interestingQuestion as i using (question_id) 
group by q.category_id 

这是给我下面的结果 - 就像我需要根据我在我的表中的数据：

Category_id Count 

    5   1 
    6   3 

现在我需要找到具有最高计数器CATEGORY_ID，所以我做了以下查询：

SELECT t.Category_id, MAX(t.Count) 
from(
    SELECT q.category_id as Category_id , COUNT(q.question_id) as count 
    from questions as q INNER JOIN interestingQuestion as i using (question_id) 
    group by q.category_id 
)as t 

和那我得到的结果是：

category_id MAX(t.count) 
    5    3 

这是一个混合起来的结果，它的发现最大计数器，但它给我一个错误的CATEGORY_ID

为什么会发生？我该如何解决它？

Answer 1

您可以使用此：

SELECT 
    q.category_id as Category_id, 
    COUNT(q.question_id) as count 
FROM 
    questions as q INNER JOIN interestingQuestion as i 
    USING (question_id) 
GROUP BY q.category_id 
ORDER BY 
    COUNT(q.question_id) DESC 
LIMIT 1 

这将COUNT在递减顺序，只返回包含您需要的值的第一行订购您的结果。

编辑

如果存在具有相同的最大值多个行，你可以使用这样的事情：

SELECT 
    q.category_id as Category_id, 
    COUNT(q.question_id) as count 
FROM 
    questions as q INNER JOIN interestingQuestion as i 
    USING (question_id) 
GROUP BY 
    q.category_id 
HAVING 
    COUNT(q.question_id) = (SELECT MAX(t.Count) FROM (
    SELECT COUNT(q.question_id) as count 
    FROM 
     questions as q INNER JOIN interestingQuestion as i 
     USING (question_id) 
    GROUP BY 
     q.category_id) as t) 

我用你的查询作为子查询来计算最大计数，并且我返回所有行HAVING COUNT（）=（查询返回的最大值）。

Answer 2

这应该工作

SELECT q.category_id as Category_id , COUNT(q.question_id) as count 
from questions as q INNER JOIN interestingQuestion as i using (question_id) 
group by q.category_id HAVING max(count(q.question_id)) 

，但你可能需要做这种方式。

SELECT t.Category_id, t.Count 
from(
    SELECT q.category_id as Category_id , COUNT(q.question_id) as count 
    from questions as q INNER JOIN interestingQuestion as i using (question_id) 
    group by q.category_id 
)as t 
order by max(t.count) desc 
limit 1