获取组内表中的前 n 个计数行 [MySQL]

Question

我正在尝试仅获得按类别分组的前 3 名销售产品（按每个类别在交易 (id) count(id) 中出现的前 3 名产品）。我一直在寻找可能的解决方案，但没有结果。看起来它在 MySQL 中有点棘手，因为不能简单地使用 top() 函数等等。示例数据结构如下：

+--------+------------+-----------+
|     id |category_id | product_id|
+--------+------------+-----------+
| 1      | 10         | 32        |
| 2      | 10         | 34        |
| 3      | 10         | 32        |
| 4      | 10         | 21        |
| 5      | 10         | 100       |
| 6      | 7          | 101       |
| 7      | 7          | 39        |
| 8      | 7          | 41        |
| 9      | 7          | 39        |
+--------+------------+-----------+

Answer 1

如果你运行的是 MySQL 8.0，你可以使用窗口函数 rank() 来实现：

select *
from (
    select 
        category_id,
        product_id,
        count(*) cnt,
        rank() over(partition by category_id order by count(*) desc) rn
    from mytable
    group by category_id, product_id
) t
where rn <= 3

在早期版本中，一种选择是使用相关子查询进行过滤：

select 
    category_id,
    product_id,
    count(*) cnt
from mytable t
group by category_id, product_id
having count(*) >= (
    select count(*)
    from mytable t1
    where t1.category_id = t.category_id and t1.product_id = t.product_id
    order by count(*) desc
    limit 3, 1
)

Answer 2

在早期版本的 MySQL 中，我建议使用变量：

select cp.*
from (select cp.*,
             (@rn := if(@c = category_id, @rn + 1,
                        if(@c := category_id, 1, 1)
                       )
             ) as rn
      from (select category_id, product_id, count(*) as cnt
            from mytable
            group by category_id, product_id
            order by category_id, count(*) desc
           ) cp cross join
           (select @c := -1, @rn := 0) params
     ) cp
where rn <= 3;