MYSQL - 按限制分组

Question

有没有一种简单的方法将GROUP BY结果限制在前2位.以下查询返回所有结果.使用"LIMIT 2"仅将整个列表减少到前2个条目.

select distinct(rating_name), 
       id_markets, 
       sum(rating_good) 'good', 
       sum(rating_neutral)'neutral', 
       sum(rating_bad) 'bad' 
 from ratings 
 where rating_year=year(curdate()) and rating_week= week(curdate(),1)
 group by rating_name,id_markets
 order by rating_name, sum(rating_good) 
 desc

结果如下: -

poland  78 48 24 12   <- keep
poland   1 15  5  0   <- keep
poland  23 12  6  3
poland   2  5  0  0
poland   3  0  5  0
poland   4  0  0  5
ireland  1  9  3  0   <- keep
ireland  2  3  0  0   <- keep
ireland  3  0  3  0
ireland  4  0  0  3
france  12 24 12  6   <- keep
france   1  3  1  0   <- keep
france 231  1  0  0
france   2  1  0  0
france   4  0  0  1
france   3  0  1  0

谢谢乔恩

---

根据要求,我附上了表格结构和一些测试数据的副本.我的目标是创建一个包含每个唯一rating_name的前2个结果的视图

CREATE TABLE `zzratings` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `id_markets` int(11) DEFAULT NULL,
  `id_account` int(11) DEFAULT NULL,
  `id_users` int(11) DEFAULT NULL,
  `dateTime` timestamp NULL DEFAULT CURRENT_TIMESTAMP,
  `rating_good` int(11) DEFAULT NULL,
  `rating_neutral` int(11) DEFAULT NULL,
  `rating_bad` int(11) DEFAULT NULL,
  `rating_name` varchar(32) DEFAULT NULL,
  `rating_year` smallint(4) DEFAULT NULL,
  `rating_week` tinyint(4) DEFAULT NULL,
  `cash_balance` decimal(9,6) DEFAULT NULL,
  `cash_spend` decimal(9,6) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `rating_year` (`rating_year`),
  KEY `rating_week` (`rating_week`),
  KEY `rating_name` (`rating_name`)
) ENGINE=MyISAM AUTO_INCREMENT=2166690 DEFAULT CHARSET=latin1;

INSERT INTO `zzratings` (`id`,`id_markets`,`id_account`,`id_users`,`dateTime`,`rating_good`,`rating_neutral`,`rating_bad`,`rating_name`,`rating_year`,`rating_week`,`cash_balance`,`cash_spend`)
VALUES
    (63741, 1, NULL, 100, NULL, 1, NULL, NULL, 'poland', 2010, 15, NULL, NULL),
    (63742, 1, NULL, 101, NULL, 1, NULL, NULL, 'poland', 2010, 15, NULL, NULL),
    (1, 2, NULL, 102, NULL, 1, NULL, NULL, 'poland', 2010, 15, NULL, NULL),
    (63743, 3, NULL, 103, NULL, NULL, 1, NULL, 'poland', 2010, 15, NULL, NULL),
    (63744, 4, NULL, 104, NULL, NULL, NULL, 1, 'poland', 2010, 15, NULL, NULL),
    (63745, 1, NULL, 105, NULL, 1, NULL, NULL, 'poland', 2010, 15, NULL, NULL),
    (63746, 1, NULL, 106, NULL, NULL, 1, NULL, 'poland', 2010, 15, NULL, NULL),
    (63747, 5, NULL, 100, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
    (63748, 5, NULL, 101, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
    (63749, 2, NULL, 102, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
    (63750, 3, NULL, 103, NULL, NULL, 1, NULL, 'ireland', 2010, 15, NULL, NULL),
    (63751, 4, NULL, 104, NULL, NULL, NULL, 1, 'ireland', 2010, 15, NULL, NULL),
    (63752, 1, NULL, 105, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
    (63753, 1, NULL, 106, NULL, NULL, 1, NULL, 'ireland', 2010, 15, NULL, NULL),
    (63754, 1, NULL, 100, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
    (63755, 1, NULL, 101, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
    (63756, 2, NULL, 102, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
    (63757, 34, NULL, 103, NULL, NULL, 1, NULL, 'ireland', 2010, 15, NULL, NULL),
    (63758, 34, NULL, 104, NULL, NULL, NULL, 1, 'ireland', 2010, 15, NULL, NULL),
    (63759, 34, NULL, 105, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
    (63760, 34, NULL, 106, NULL, NULL, 1, NULL, 'ireland', 2010, 15, NULL, NULL),
    (63761, 21, NULL, 100, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
    (63762, 21, NULL, 101, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
    (63763, 21, NULL, 102, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
    (63764, 21, NULL, 103, NULL, NULL, 1, NULL, 'ireland', 2010, 15, NULL, NULL),
    (63765, 4, NULL, 104, NULL, NULL, NULL, 1, 'ireland', 2010, 15, NULL, NULL),
    (63766, 1, NULL, 105, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
    (63767, 1, NULL, 106, NULL, NULL, 1, NULL, 'ireland', 2010, 15, NULL, NULL),
    (63768, 1, NULL, 100, NULL, 1, NULL, NULL, 'france', 2010, 15, NULL, NULL),
    (63769, 1, NULL, 101, NULL, 1, NULL, NULL, 'france', 2010, 15, NULL, NULL),
    (63770, 2, NULL, 102, NULL, 1, NULL, NULL, 'france', 2010, 15, NULL, NULL),
    (63771, 3, NULL, 103, NULL, NULL, 1, NULL, 'france', 2010, 15, NULL, NULL),
    (63772, 4, NULL, 104, NULL, NULL, NULL, 1, 'france', 2010, 15, NULL, NULL);

Answer 1

我不认为MySQL中有一种简单的方法.一种方法是通过按rating_name为组划分的每一行生成行号,然后仅选择row_number为2或更小的行.在大多数数据库中,您可以使用以下内容执

SELECT * FROM (
    SELECT
        rating_name,
        etc...,
        ROW_NUMBER() OVER (PARTITION BY rating_name ORDER BY good) AS rn
    FROM your_table
) T1
WHERE rn <= 2

不幸的是,MySQL不支持ROW_NUMBER语法.但是,您可以ROW_NUMBER使用变量进行模拟:

SELECT
    rating_name, id_markets, good, neutral, bad
FROM (
    SELECT
        *,
        @rn := CASE WHEN @prev_rating_name = rating_name THEN @rn + 1 ELSE 1 END AS rn,
        @prev_rating_name := rating_name
    FROM (
        SELECT
            rating_name,
            id_markets,
            SUM(COALESCE(rating_good, 0)) AS good,
            SUM(COALESCE(rating_neutral, 0)) AS neutral,
            SUM(COALESCE(rating_bad, 0)) AS bad
        FROM zzratings
        WHERE rating_year = YEAR(CURDATE()) AND rating_week = WEEK(CURDATE(), 1)
        GROUP BY rating_name, id_markets
    ) AS T1, (SELECT @prev_rating_name := '', @rn := 0) AS vars
    ORDER BY rating_name, good DESC
) AS T2
WHERE rn <= 2
ORDER BY rating_name, good DESC

运行测试数据时的结果:

france    1  2  0  0
france    2  1  0  0
ireland   1  4  2  0
ireland  21  3  1  0
poland    1  3  1  0
poland    2  1  0  0