MySql分组后随机获取每组一条数据的操作

思路：先随机排序然后再分组就好了。

1、创建表：

CREATE TABLE `xdx_test` ( `id` int(11) NOT NULL, `name` varchar(255) DEFAULT NULL, `class` varchar(255) DEFAULT NULL, PRIMARY KEY (`id`)) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

INSERT INTO xdx_test VALUES (1, '张三-1','1');INSERT INTO xdx_test VALUES (2, '李四-1','1');INSERT INTO xdx_test VALUES (3, '王五-1','1');INSERT INTO xdx_test VALUES (4, '张三-2','2');INSERT INTO xdx_test VALUES (5, '李四-2','2');INSERT INTO xdx_test VALUES (6, '王五-2','2');INSERT INTO xdx_test VALUES (7, '张三-3','3');INSERT INTO xdx_test VALUES (8, '李四-3','3');INSERT INTO xdx_test VALUES (9, '王五-3','3');

SELECT * FROM  (SELECT * FROM xdx_test ORDER BY RAND()) aGROUP BY a.class

3 王五-1 1
5 李四-2 2
9 王五-3 3

3 王五-1 1
4 张三-2 2
7 张三-3 3

2 李四-1 1
5 李四-2 2
8 李四-3 3

补充知识：mysql实现随机获取几条数据的方法（效率和离散型比较）

sql语句有几种写法、效率、以及离散型比较

1：SELECT * FROM tablename ORDER BY RAND() LIMIT 想要获取的数据条数；

2：SELECT *FROM `table` WHERE id >= (SELECT FLOOR( MAX(id) * RAND()) FROM `table` ) ORDER BY id LIMIT 想要获取的数据条数;

3：SELECT * FROM `table` AS t1 JOIN (SELECT ROUND(RAND() * (SELECT MAX(id) FROM `table`)) AS id) AS t2 WHERE t1.id >= t2.id

ORDER BY t1.id ASC LIMIT 想要获取的数据条数;

4：SELECT * FROM `table`WHERE id >= (SELECT floor(RAND() * (SELECT MAX(id) FROM `table`))) ORDER BY id LIMIT 想要获取的数据条数;

5：SELECT * FROM `table` WHERE id >= (SELECT floor( RAND() * ((SELECT MAX(id) FROM `table`)-(SELECT MIN(id) FROM `table`)) + (SELECT MIN(id) FROM `table`))) ORDER BY id LIMIT 想要获取的数据条数;

6：SELECT * FROM `table` AS t1 JOIN (SELECT ROUND(RAND() * ((SELECT MAX(id) FROM `table`)-(SELECT MIN(id) FROM `table`))+(SELECT MIN(id) FROM `table`)) AS id) AS t2 WHERE t1.id >= t2.id ORDER BY t1.id LIMIT 想要获取的数据条数;

1的查询时间>>2的查询时间>>5的查询时间>6的查询时间>4的查询时间>3的查询时间，也就是3的效率最高。

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