SQL max函數

Question

我有以下查詢：

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 

這是給我下面的結果 - 就像我需要根據我在我的表中的數據：

Category_id Count 

    5   1 
    6   3 

現在我需要找到具有最高計數器CATEGORY_ID，所以我做了以下查詢：

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 

和那我得到的結果是：

category_id MAX(t.count) 
    5    3 

這是一個混合起來的結果，它的發現最大計數器，但它給我一個錯誤的CATEGORY_ID

爲什麼會發生？我該如何解決它？

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 

這將COUNT在遞減順序，只返回包含您需要的值的第一行訂購您的結果。

編輯

如果存在具有相同的最大值多個行，你可以使用這樣的事情：

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) 

我用你的查詢作爲子查詢來計算最大計數，並且我返回所有行HAVING COUNT（）=（查詢返回的最大值）。

Answer 2

這應該工作

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)) 

，但你可能需要做這種方式。

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