MySQL中取字段数据前N条记录
表格：
Id | category_id | created_at
------ | ----------- ----------
1 | 1 | 2017-12-01
2 | 2 | 2017-12-02
3 | 4 | 2017-12-03
4 | 2 | 2017-12-04
5 | 1 | 2017-12-05
6 | 1 | 2017-12-06
7 | 3 | 2017-12-07
8 | 4 | 2017-12-08
9 | 4 | 2017-12-09
10 | 3 | 2017-12-10
11 | 5 | 2017-12-11
预期结果：
Ids
------
5
6
2
4
7
10
8
9
11
查询语句：
SELECT *
FROM
(SELECT *,
@count := IF(@current_category = category_id, @count + 1, 1) AS count,
@current_category := category_id
FROM table
ORDER BY category_id, created_at DESC
) ranked
WHERE count <= 2;