关于mysql:mysqlgroup-by-组内排序

测试数据

CREATE TABLE `book_comment` (`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `user_id` int(10) unsigned NOT NULL,
  `like_num` int(10) NOT NULL DEFAULT '0' COMMENT '点赞数',
  `content` varchar(200) NOT NULL,
  `addtime` datetime NOT NULL,
  PRIMARY KEY (`id`),
  KEY `user_id` (`user_id`),
  KEY `addtime` (`addtime`),
  KEY `uid_addtime` (`user_id`,`addtime`)
) ENGINE=InnoDB;


INSERT INTO `book_comment` VALUES ('1', '1', '0', '评论 1', '2017-05-17 00:00:00');
INSERT INTO `book_comment` VALUES ('2', '1', '2', '评论 2', '2017-05-17 00:03:01');
INSERT INTO `book_comment` VALUES ('3', '2', '3', '评论 3', '2017-05-17 00:03:02');
INSERT INTO `book_comment` VALUES ('4', '2', '4', '评论 4', '2017-05-17 00:00:03');
INSERT INTO `book_comment` VALUES ('5', '3', '0', '评论 5', '2017-05-17 00:00:04');
INSERT INTO `book_comment` VALUES ('6', '1', '1', '评论 6', '2017-05-17 00:00:05');
INSERT INTO `book_comment` VALUES ('7', '4', '0', '评论 7', '2017-05-17 00:00:06');
INSERT INTO `book_comment` VALUES ('8', '4', '2', '评论 8', '2017-05-17 00:00:07');
INSERT INTO `book_comment` VALUES ('9', '4', '0', '评论 9', '2017-05-17 00:00:08');
INSERT INTO `book_comment` VALUES ('10', '4', '3', '评论 10', '2017-05-17 00:00:09');
INSERT INTO `book_comment` VALUES ('11', '3', '2', '评论 11', '2017-05-17 00:00:10');

1：惯例 group 语句

mysql> SELECT *,count(*) as num from book_comment GROUP BY user_id;
+----+---------+----------+---------+---------------------+-----+
| id | user_id | like_num | content | addtime             | num |
+----+---------+----------+---------+---------------------+-----+
|  1 |       1 |        0 | 评论 1   | 2017-05-17 00:00:00 |   3 |
|  3 |       2 |        3 | 评论 3   | 2017-05-17 00:03:02 |   2 |
|  5 |       3 |        0 | 评论 5   | 2017-05-17 00:00:04 |   2 |
|  7 |       4 |        0 | 评论 7   | 2017-05-17 00:00:06 |   4 |
+----+---------+----------+---------+---------------------+-----+

依据用户分组，分组内默认显示第一条数据

2：显示分组中最新的数据

mysql> SELECT *,count(*) as num from book_comment GROUP BY user_id desc;
+----+---------+----------+---------+---------------------+-----+
| id | user_id | like_num | content | addtime             | num |
+----+---------+----------+---------+---------------------+-----+
| 10 |       4 |        3 | 评论 10  | 2017-05-17 00:00:09 |   4 |
| 11 |       3 |        2 | 评论 11  | 2017-05-17 00:00:10 |   2 |
|  4 |       2 |        4 | 评论 4   | 2017-05-17 00:00:03 |   2 |
|  6 |       1 |        1 | 评论 6   | 2017-05-17 00:00:05 |   3 |
+----+---------+----------+---------+---------------------+-----+

3：减少显示用户最大点赞数

mysql> SELECT *,count(*),max(like_num) as num from book_comment GROUP BY user_id desc;
+----+---------+----------+---------+---------------------+----------+-----+
| id | user_id | like_num | content | addtime             | count(*) | num |
+----+---------+----------+---------+---------------------+----------+-----+
| 10 |       4 |        3 | 评论 10  | 2017-05-17 00:00:09 |        4 |   3 |
| 11 |       3 |        2 | 评论 11  | 2017-05-17 00:00:10 |        2 |   2 |
|  4 |       2 |        4 | 评论 4   | 2017-05-17 00:00:03 |        2 |   4 |
|  6 |       1 |        1 | 评论 6   | 2017-05-17 00:00:05 |        3 |   2 |
+----+---------+----------+---------+---------------------+----------+-----+

4: 获取不同用户点赞数最多的那一行

mysql> SELECT
    a.*
FROM
    book_comment AS a
INNER JOIN (
    SELECT
        user_id,
        max(like_num) AS like_num
    FROM
        book_comment
    GROUP BY
        user_id
) AS b ON a.user_id = b.user_id
AND a.like_num = b.like_num
ORDER BY
    a.user_id ASC;

+----+---------+----------+---------+---------------------+
| id | user_id | like_num | content | addtime             |
+----+---------+----------+---------+---------------------+
|  2 |       1 |        2 | 评论 2   | 2017-05-17 00:03:01 |
|  4 |       2 |        4 | 评论 4   | 2017-05-17 00:00:03 |
| 11 |       3 |        2 | 评论 11  | 2017-05-17 00:00:10 |
| 10 |       4 |        3 | 评论 10  | 2017-05-17 00:00:09 |
+----+---------+----------+---------+---------------------+

5: 其余分组排序办法

利用窗口函数性能 (mysql8 版本新增) 实现分组排序功能