mysql分组随机取数据_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 33 王五-1 1
4 张三-2 2
7 张三-3 32 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',
mysql删除重复的数据保留一条`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_aming_type,t1.interface,t1.xdr_id,t1.rat,t1.imsi,t1.i
' into
@id,@length,@local_province,@local_city,@owner_province,@owner_city,@roaming_type,@interface,@xdr_id,@rat,@imsi,@imei,@ms ',
' 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,pro
values(@id,@length,@local_province,@local_city,@owner_province,@owner_city,@roaming_type,@interface,@xdr_id,@rat,@imsi,
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_tim from xdr_authen_tmp;
end;
truncate table xdr_authen_tmp;
end
;;
delimiter ;
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