每组获得N个MYSQL

Question

问题不在于 ： 在GROUP BY中使用LIMIT获得每个组N个结果？

但我承认这是相似的。

我需要选择每人前2行。 行按收到年份排序

问题：同一个月可能输入了2个数据（输入的日期为YYYY-MM）

我附带的查询（在提到的问题之后）被卡在BIG循环中。

SELECT *
FROM `table_data` as b
WHERE (
    SELECT count(*) FROM `table_data` as a
        WHERE a.Nom = b.Nom and a.year < b.year
    ) <= 2;

样本数据 ：

  A  |   year   |  Nom
---------------------
  b  |  2011-01 | Tim
---------------------
  d  |  2011-01 | Tim
---------------------
  s  |  2011-01 | Tim
---------------------
  a  |  2011-03 | Luc
---------------------
  g  |  2011-01 | Luc
---------------------
  s  |  2011-01 | Luc

应出口：

  A  |   year   |  Nom
---------------------
  b  |  2011-01 | Tim
---------------------
  d  |  2011-01 | Tim
---------------------
  a  |  2011-03 | Luc
---------------------
  g  |  2011-01 | Luc

Answer 1

(
 -- First get a set of results as if you only wanted the latest entry for each
 -- name - a simple GROUP BY from a derived table with an ORDER BY
  SELECT *
  FROM (
    SELECT *
    FROM `table_data`
    ORDER BY `year` DESC
  ) `a`
  GROUP BY `Nom`
)
UNION
(
 -- Next union it with the set of result you get if you apply the same criteria
 -- and additionally specify that you do not want any of the rows found by the
 -- first operation
  SELECT *
  FROM (
    SELECT *
    FROM `table_data`
    WHERE `id` NOT IN (
      SELECT `id`
      FROM (
        SELECT *
        FROM `table_data`
        ORDER BY `year` DESC
      ) `a`
      GROUP BY `Nom`
    )
    ORDER BY `year` DESC
  ) `b`
  GROUP BY `Nom`
)
 -- Optionally apply ordering to the final results
ORDER BY `Nom` DESC, `year` DESC

我确信可以用一种更短的方法来完成此操作，但是现在我无法一生解决它的问题。 但这确实有效 -假设您有一个主键（应有）并且它称为id 。