解决group_by和oder_by同时使用无效的问题

数据表

CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) NOT NULL,
  `score` int(11) NOT NULL DEFAULT '20' COMMENT '年龄',
  `create_date` timestamp NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
# 数据
insert into `student` (`id`, `name`, `score`, `create_date`) values('1','张三','11','2018-11-19 16:29:26');
insert into `student` (`id`, `name`, `score`, `create_date`) values('2','李四','21','2018-11-19 16:29:26');
insert into `student` (`id`, `name`, `score`, `create_date`) values('3','王五','31','2018-11-19 16:29:26');
insert into `student` (`id`, `name`, `score`, `create_date`) values('4','张三','12','2018-11-20 16:34:12');
insert into `student` (`id`, `name`, `score`, `create_date`) values('5','李四','22','2018-11-20 16:33:34');
insert into `student` (`id`, `name`, `score`, `create_date`) values('6','王五','32','2018-11-20 16:33:34');

问题： 如果我想按照name分组，并且按照时间获取最新的数据

SELECT * FROM test.student GROUP BY name ORDER BY create_date DESC;

这样的话，无法获取score为12,22,32的数据

方案： 采用group_concat() + substring_index()函数来解决问题

函数group_concat

# 分隔符 默认是 ,
group_concat([DISTINCT] 要连接的字段 [Order BY 排序字段 ASC/DESC] [Separator '分隔符'])

例如：SELECT name , GROUP_CONCAT(score ORDER BY create_date DESC SEPARATOR ',') scores FROM test.student GROUP BY name;

函数substring_index

substring_index(str,delim,count)
例：SELECT SUBSTRING_INDEX('blog.jb51.net', '.' ,2);
结果：blog.jb51

sql语句

SELECT SUBSTRING_INDEX(GROUP_CONCAT(id ORDER BY create_date DESC), ',', 1) id, name, SUBSTRING_INDEX(GROUP_CONCAT(score ORDER BY create_date DESC), ',', 1) score FROM test.student GROUP BY name;