Question

我到处都是。除了两个问题之外，下面的MySql查询适用于我。

1）我想确保每个类别只显示一个结果（我有5个类别），我尝试使用DISTINCT但明显误解了那个。

2）我希望能够在sql查询之外返回howmany值：

"SELECT DISTINCT category_id, video_id, date_added, COUNT(video_id) AS howmany
FROM votes 
WHERE date_added BETWEEN SYSDATE() - INTERVAL 7 DAY AND SYSDATE()
GROUP BY video_id 
ORDER BY howmany DESC, video_id
LIMIT 5";

我的投票表看起来像这样：

id | video_id | category_id | date_added

编辑：预期产出 -

video_id 2 category_id 4投票数500

video_id 5 category_id 1投票数377

video_id 88 category_id 3投票数25

video_id 45 category_id 5投票数23

video_id 9 category_id 2投票数2

显示过去7天内每个类别中投票最多的视频。

Answer 1

如果你想进一步打破结果，很可能你会想GROUP BY category_id，甚至GROUP BY category_id, video_id。

Answer 2

如果每个类别只需要1个结果，则必须按类别分组。但由于每个类别可能有多个video_id，因此除非使用GROUP_CONCAT，否则最终会从类别中获取一个随机视频。

如果您只想获得列值的明确计数，请将DISTINCT关键字放在count函数中：

COUNT( DISTINCT video_id )

Answer 3

SELECT DISTINCT category_id, video_id, date_added, COUNT(video_id) AS howmany

在功能上等同于

GROUP BY category_id, video_id, date_added, COUNT(video_id)

你说“我想确保每个类别只显示一个结果”，所以你可能想要这样的东西：

SELECT category_id, video_id, date_added, COUNT(video_id) AS howmany
FROM votes 
WHERE date_added BETWEEN SYSDATE() - INTERVAL 7 DAY AND SYSDATE()
GROUP BY category_id 
ORDER BY howmany DESC, video_id
LIMIT 5;

Answer 4

MySQL没有您用于这类情况的分析功能 - 使用：

SELECT x.video_id,
       x.category_id,
       x.howmany
  FROM (SELECT t.video_id,
               t.category_id,
               COUNT(*) AS howmany,
               CASE 
                 WHEN @category = t.category_id THEN @rownum 
                 ELSE @rownum := @rownum + 1 
               END AS rank,
               @category := t.category_id
          FROM VOTES t
          JOIN (SELECT @rownum := 0, @category := -1) r
         WHERE t.date_added BETWEEN SYSDATE() - INTERVAL 7 DAY AND SYSDATE()
      GROUP BY t.video_id, t.category_id
     ORDER BY t.category_id, howmany DESC) x
 WHERE x.rank = 1
ORDER BY x.howmany DESC
   LIMIT 5

mysql对count的不同值

4 个答案: