php - 如何找到所需的唯一结果

Question

我有两个表“主题”和“子类别”

我正在使用这个查询——

Select * from `subcategory` as s 
Inner join `topic` as f
WHERE s.`Subcategory_id` = f.`Subcategory_id

我的结果显示像

Category_id Subcategory_id Post_id 时间

    2        2.3         4    2012-12-01
    1        1.5         5    2013-01-20
    1        1.3         6    2013-03-18

还有其他列......但我想要的只是选择一个 Category_id 的最新 Post_id 和 Subcategory_id ...这意味着这里的 Category 1 有两个 Subcategory 它只会选择最新的（这里是 1.3）和相同的结果数据库将变大时的所有类别。下一个查询是什么，或者我如何更改现有查询以获得我想要的结果？

Answer 1

SELECT TOP 1 ... ORDER BY whatever column determines "the latest"

例如

SELECT TOP 1 ... ORDER BY TIME DESCENDING

或者在mysql的情况下：

SELECT ... ORDER BY TIME DESCENDING LIMIT 1

Answer 2

SELECT Post_Id, Subcategory_Id from subcategory as s, topic as t where 
   s.Subcategory_id = t.Subcategory_id and time = (
      SELECT Max(time) from subcategory as s1, topic as t1 where 
         s1.Subcategory_id = t1.Subcategory_id and s1.Category_id = s.Category_id
   );

我认为，这样的事情会奏效。

Answer 3

使用以下查询加入您的主题表：

SELECT s.* FROM subcategory s
    Inner JOIN (SELECT s1.Category_id,
                 MAX(s1.time1) AS max_time
                 FROM subcategory s1
                 GROUP BY s1.Category_id) y 
ON y.Category_id = s.Category_id AND y.max_time = s.time1