mysql选择每组前n行 [英] mysql select the first n rows per group

问题描述

我读过这篇文章： http://www.xaprb.com/blog/2006/12/07/how-to-select-the-firstleastmax-row-per-group-in-sql/ 并搜索其他问题

I've read this article: http://www.xaprb.com/blog/2006/12/07/how-to-select-the-firstleastmax-row-per-group-in-sql/ and search for other questions

我有一个类似于以下内容的表：

I have a table that is something like:

| table.id    | USER.id
----------------------------------------------
| 1           | 101
| 2           | 101
| 3           | 101
| 4           | 101 
| 5           | 101
| 6           | 101
| 7           | 101 
| 8           | 101
| 9           | 101 
| 10          | 101
| 11          | 102
| 12          | 102 
| 13          | 102
| 14          | 102
| 15          | 103 
| 16          | 103
| 17          | 103
| 18          | 103
| 19          | 103
| 20          | 103
| 21          | 103
| 22          | 103 
| 23          | 103
| 24          | 104
| 25          | 104 
| 26          | 104
| 27          | 104
| 28          | 104
| 29          | 104
| 30          | 105
| 31          | 105
| 32          | 105 
| 33          | 106
| 34          | 106

我试图获取由user.id分组的table.id数量，如果user.id的计数大于7，只显示结果为7（也就是将计数结果限制为7）。

I'm trying to get the count of table.id grouped by user.id, and if the count of user.id is more than 7, only display the result as 7 (aka limiting the count results to 7).

在这个例子中，结果应该是：

In this example, the result should be:

| USER.id         | count of table.ID
----------------------------------------
| 101             | 7
| 102             | 4
| 103             | 7
| 104             | 6
| 105             | 3
| 106             | 2

我试过了：

I've tried:

SELECT USERid, COUNT(table.id)
FROM table
WHERE table.id IN (select top 7 table.id from table)
GROUP BY USERid

和

SELECT USERid, COUNT(table.id)
FROM table
WHERE (
    SELECT COUNT(table.ID) FROM table as t
    WHERE t.id = t.id AND t.USERid <= table.USERid
    ) <= 7
GROUP BY USERid

推荐答案

您可以简化查询，并使用 LEAST 函数

You can simplify your query, and use LEAST function

SELECT USERid, LEAST(7, COUNT(*))
FROM table
GROUP BY USERid

您的评论

from the question in your comment

SELECT SUM(countByUser) 
FROM
 (SELECT LEAST(7, COUNT(*)) as countByUser
  FROM table
  GROUP BY USERid) c

SqlFiddle

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