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;
2、插⼊数据
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');
3、查询语句
SELECT * FROM
(SELECT * FROM xdx_test ORDER BY RAND()) a
GROUP BY a.class
4、查询结果
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。
以上6种只是单纯的从效率上做了⽐较；
上⾯的6种随机数抽取可分为2类：
第⼀个的离散型⽐较⾼，但是效率低；其他5个都效率⽐较⾼，但是存在离散性不⾼的问题；
怎么解决效率和离散型都满⾜条件啦？
我们有⼀个思路就是：写⼀个存储过程；
select * FROM test t1 JOIN (SELECT ROUND(RAND() * ((SELECT MAX(id) FROM test)-(SELECT MIN(id) FROM test)) + (SELECT MIN(id) FROM test)) AS id) t2 where t1.id >= t2.id limit 1
每次取出⼀条，然后循环写⼊⼀张临时表中；最后返回 select 临时表就OK；
这样既满⾜了效率⼜解决了离散型的问题；可以兼并⼆者的优点；
下⾯是具体存储过程的伪代码
DROP PROCEDURE IF EXISTS `evaluate_Check_procedure`;
DELIMITER ;;
CREATE DEFINER=`root`@`%` PROCEDURE `evaluate_Check_procedure`(IN startTime datetime, IN endTime datetime,IN checkNum INT,IN evaInterface VARCHAR(36))
BEGIN
-- 新建⼀张临时表 ，存放随机取出的数据
create temporary table if not exists xdr_authen_tmp (
`ID` bigint(20) NOT NULL AUTO_INCREMENT COMMENT '序号',
`LENGTH` int(5) DEFAULT NULL COMMENT '字节数',
`INTERFACE` int(3) NOT NULL COMMENT '接⼝',
`XDR_ID` varchar(32) NOT NULL COMMENT 'XDR ID',
`MSISDN` varchar(32) DEFAULT NULL COMMENT '⽤户号码',
`PROCEDURE_START_TIME` datetime NOT NULL DEFAULT '0000-00-00 00:00:00' COMMENT '开始时间',
`PROCEDURE_END_TIME` datetime DEFAULT NULL COMMENT '结束时间',
`SOURCE_NE_IP` varchar(39) DEFAULT NULL COMMENT '源⽹元IP',
`SOURCE_NE_PORT` int(5) DEFAULT NULL COMMENT '源⽹元端⼝',
`DESTINATION_NE_IP` varchar(39) DEFAULT NULL COMMENT '⽬的⽹元IP',
`DESTINATION_NE_PORT` int(5) DEFAULT NULL COMMENT '⽬的⽹元端⼝',
`INSERT_DATE` datetime DEFAULT NULL COMMENT '插⼊时间',
`EXTEND1` varchar(50) DEFAULT NULL COMMENT '扩展1',
`EXTEND2` varchar(50) DEFAULT NULL COMMENT '扩展2',
`EXTEND3` varchar(50) DEFAULT NULL COMMENT '扩展3',
`EXTEND4` varchar(50) DEFAULT NULL COMMENT '扩展4',
`EXTEND5` varchar(50) DEFAULT NULL COMMENT '扩展5',
PRIMARY KEY (`ID`,`PROCEDURE_START_TIME`),
KEY `index_procedure_start_time` (`PROCEDURE_START_TIME`),
KEY `index_source_dest_ip` (`SOURCE_NE_IP`,`DESTINATION_NE_IP`),
KEY `index_xdr_id` (`XDR_ID`)
) ENGINE = InnoDB DEFAULT CHARSET=utf8;
BEGIN
DECLARE j INT;
DECLARE i INT;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET i = 1;
-- 这⾥的checkNum是需要随机获取的数据数，⽐如随机获取10条，那这⾥就是10，通过while循环来逐个获取单个随机记录；
SET j = 0;
WHILE j < checkNum DO
set @sqlexi = concat( ' SELECT t1.ID,t1.LENGTH,t1.LOCAL_PROVINCE,t1.LOCAL_CITY,t1.OWNER_PROVINCE,t1.OWNER_CITY,t1.ROAMING_TYPE,t1.INTERFACE,t1.XDR_ID,t1.RAT,t1.IMSI,t1.IMEI,t1.MSISDN,t1.PROCEDURE_START_TIME,t1.PRO    ' into @ID,@LENGTH,@LOCAL_PROVINCE,@LOCAL_CITY,@OWNER_PROVINCE,@OWNER_CITY,@ROAMING_TYPE,@INTERFACE,@XDR_ID,@RAT,@IMSI,@IMEI,@MSISDN,@PROCEDURE_START_TIME,@PROCEDURE_END_TIME,@TRAN    ' FROM xdr_authen t1 JOIN (SELECT ROUND(RAND() * ((SELECT MAX(id) FROM xdr_authen)-(SELECT MIN(id) FROM xdr_authen)) + (SELECT MIN(id) FROM xdr_authen)) AS id) t2',
' WHERE t1.PROCEDURE_START_TIME >= "',startTime,'"',
' AND t1.PROCEDURE_START_TIME < "',endTime,'"',' AND t1.INTERFACE IN (',evaInterface,')',
' and t1.id >= t2.id limit 1');
PREPARE sqlexi FROM @sqlexi;
EXECUTE sqlexi;
DEALLOCATE PREPARE sqlexi;
-- 这⾥获取的记录有可能会重复，如果是重复数据，我们则不往临时表中插⼊此条数据，再进⾏下⼀次随机数据的获取。依次类推，直到随机数据取够为⽌；
select count(1) into @num from xdr_authen_tmp where id = @ID;
if @num > 0 or i=1 then
SET j = j;
ELSE
insert into xdr_authen_tmp(ID,LENGTH,LOCAL_PROVINCE,LOCAL_CITY,OWNER_PROVINCE,OWNER_CITY,ROAMING_TYPE,INTERFACE,XDR_ID,RAT,IMSI,IMEI,MSISDN,PROCEDURE_START_TIME,PROCEDURE_END_TIME,TRANSACTION_TYP  VALUES(@ID,@LENGTH,@LOCAL_PROVINCE,@LOCAL_CITY,@OWNER_PROVINCE,@OWNER_CITY,@ROAMING_TYPE,@INTERFACE,@XDR_ID,@RAT,@IMSI,@IMEI,@MSISDN,@PROCEDURE_START_TIME,@PROCEDURE_END_TIME,@TR  SET j = j + 1;
end if;
SET i=0;
END WHILE;
-- 最后我们将所有的随机数查询出来，以结果集的形式返回给后台
select ID,LENGTH,LOCAL_PROVINCE,LOCAL_CITY,OWNER_PROVINCE,OWNER_CITY,ROAMING_TYPE,INTERFACE,XDR_ID,RAT,IMSI,IMEI,MSISDN,PROCEDURE_START_TIME,PROCEDURE_END_TIME,TRANSACTION_TYPE,TRANSACTION_ST END;
truncate TABLE xdr_authen_tmp;
END
;;
DELIMITER ;
mysql group by order by
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