sql - SQL查询输出错误

Question

我正在尝试制作一个 SQL 查询，它输出我的类别表中的每个类别。以及计算连接到类别的每个线程。现在，这很好，但是当我尝试添加一个新字段时，我需要在其中显示最新的线程日期时间，它出错了。

到目前为止，这是我的 SQL：

SELECT categories.category_name, 
       categories.category_id, 
       COUNT(threads.thread_category_id) AS 'threadCount', 
       threads.thread_datetime
FROM   categories LEFT OUTER JOIN 
       threads ON categories.category_id = threads.thread_category_id
GROUP BY categories.category_name, categories.category_id, threads.thread_datetime
ORDER BY threads.thread_datetime DESC

这导致以下结果：

但我真正希望它输出的是这个，还有一个字段，还有我的线程表中最新线程的 thread_datetime，基于 ORDER BY

我一直在尝试不同的 JOINS 以及许多其他资源。似乎找不到可行的方法。

Answer 1

您可以使用MAX()获取每个组的最新日期。

尝试以下查询：

SELECT 
    categories.category_name, 
    categories.category_id, 
    COUNT(threads.thread_category_id) AS 'threadCount', 
    MAX(threads.thread_datetime)
FROM categories 
LEFT OUTER JOIN threads 
    ON categories.category_id = threads.thread_category_id
GROUP BY categories.category_name, categories.category_id

Answer 2

试试这个（已将日期时间从您的组移动到选择的聚合部分）

SELECT categories.category_name, categories.category_id, 
COUNT(threads.thread_category_id) AS 'threadCount', MAX(threads.thread_datetime)
FROM categories LEFT OUTER JOIN threads ON categories.category_id =     threads.thread_category_id
GROUP BY categories.category_name, categories.category_id
ORDER BY max(threads.thread_datetime) DESC

Answer 3

就像是，

SELECT
                categories.category_name, 
                categories.category_id, 
                COUNT(threads.thread_category_id) AS 'threadCount'
    FROM 
                categories
        LEFT OUTER JOIN 
                threads 
                    ON categories.category_id = threads.thread_category_id
    GROUP BY 
                categories.category_name,
                categories.category_id
    ORDER BY 
                max(threads.thread_datetime) DESC