概要

` group_concat函数是mysql中非常实用的函数，它可以将同一个分组下的行拼接在一起。其完整语法：
GROUP_CONCAT([DISTINCT] 要连接的字段 [Order BY ASC/DESC 排序字段] [Separator ‘分隔符’])

使用技巧

1. 建表、插入数据

#建表语句
CREATE TABLE `test_group_concat` (
  `id` int(11) NOT NULL COMMENT 'id',
  `age` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
#插入测试数据
INSERT INTO `test`.`test_group_concat` (`id`, `age`) VALUES (1, 20);
INSERT INTO `test`.`test_group_concat` (`id`, `age`) VALUES (1, 20);
INSERT INTO `test`.`test_group_concat` (`id`, `age`) VALUES (1, 10);
INSERT INTO `test`.`test_group_concat` (`id`, `age`) VALUES (3, 30);
INSERT INTO `test`.`test_group_concat` (`id`, `age`) VALUES (3, 40);
INSERT INTO `test`.`test_group_concat` (`id`, `age`) VALUES (3, 40);
INSERT INTO `test`.`test_group_concat` (`id`, `age`) VALUES (4, 50);
INSERT INTO `test`.`test_group_concat` (`id`, `age`) VALUES (4, 60);


#基本查询
SELECT * FROM test_group_concat

2.以id分组，把age字段的值拼成一行，逗号分隔(默认)

select id,group_concat(age) from test_group_concat group by id;

3.以id分组，把age字段的值拼成 一行，分号分隔

select id,group_concat(age separator ';') from test_group_concat group by id;

4.以id分组，把去冗余的age字段的值打印在一行

select id,group_concat(distinct age) from test_group_concat group by id;

5.以id分组，把age字段的值打印在一行，逗号分隔，以age排倒序

select id,group_concat(age order by age desc) from test_group_concat group by id; 

发现问题

在项目用到如下sql

SELECT
	GROUP_CONCAT( c.goods_detail_id ) 
FROM
	air_out_order a
	LEFT JOIN air_out_order_detail b ON b.order_id = a.id
	LEFT JOIN air_out_good_record c ON c.order_detail_id = b.id; 

查询结果：

713278402284617792,722329309878140931,722329309878140949,722329309878140973,722329309878140939,715342356376936480,715342356376936482,715342356381130806,715342356376936484,715342356381130810,713278240841662516,713278240841662476,713278240841662520,715342356381130876,715342356381130940,715342356385325066,715342356385325132,715342356385325196,715342356385325260,715342356376936526,715342356381130812,715342356381130890,715342356381130964,744788201669005316,744788201669005380,744788201673199622,744788201669005330,744788201669005394,744788201752891404,744788201664811012,744788201669005344,744788201669005408,744788201752891418,744788201669005338,744788201669005410,744788201664811016,744788201669005354,744788201669005424,744788201664811030,744788201669005368,744788201673199616,744788201752891400,744788201669005312,745061736895352916,745061736891158544,745061736895352852,745061736895352930,745061736891158558,745061736895352866,745061736895352944,745061736891158572,745061736895352880,745061736895352958,74506173689115858

而实际上

SELECT
c.goods_detail_id
FROM
	air_out_order a
	LEFT JOIN air_out_order_detail b ON b.order_id = a.id
	LEFT JOIN air_out_good_record c ON c.order_detail_id = b.id;

的查询结果为：

713278402284617792
722329309878140931,722329309878140949,722329309878140973,722329309878140939
715342356376936480
715342356376936482
715342356381130806
715342356376936484
715342356381130810
713278240841662516
713278240841662476
713278240841662520
715342356381130876,715342356381130940,715342356385325066,715342356385325132,715342356385325196,715342356385325260,715342356376936526,715342356381130812,715342356381130890,715342356381130964
744788201669005316,744788201669005380,744788201673199622,744788201669005330,744788201669005394,744788201752891404,744788201664811012,744788201669005344,744788201669005408,744788201752891418
744788201669005338,744788201669005410,744788201664811016,744788201669005354,744788201669005424,744788201664811030,744788201669005368,744788201673199616,744788201752891400,744788201669005312
745061736895352916,745061736891158544,745061736895352852,745061736895352930,745061736891158558,745061736895352866,745061736895352944,745061736891158572,745061736895352880,745061736895352958,745061736891158586,745061736895352894,745061736891158536,745061736895352844,745061736895352908,745061736895352922,745061736891158550,745061736895352858,745061736895352936,745061736891158564,745061736895352872,745061736895352950,745061736891158578,745061736895352886,745061736895352964,745061736891158528,745061736895352836,745061736895352900,745061736895352914,745061736891158542
744788201669005318,744788201669005396,744788201669005334,744788201669005414,744788201669005352,744788201669005430,744788201664811022,744788201669005370,744788201752891394,744788201664811038
744788201664811020,744788201669005382,744788201752891416,744788201664811040,744788201669005400,744788201669005328,744788201669005420,744788201669005350,744788201673199618,744788201664811008
746171271546650637
746171271550844964
745061736895352842,745061736895352934,745061736895352862,745061736895352954,745061736891158548,745061736895352882,745061736891158568,745061736895352902,745061736895352832,745061736895352924
747357984740646912,747357984912613376,747357984782589952,747357984824532992,747357984652566528
747357984778395648
747357984753229824
745898989121159210,745898989108576268,745898989125353485,745898989116964869,745898989121159170,745898989121159184
745898989112770564,745898989121159168,745898989121159186,745898989121159202,745898989108576272,745898989125353481
745898989108576274,745898989116964877,745898989121159182,745898989121159206,745898989108576258,745898989116964865
745898989116964879
745898989112770562
745898989116964871,745898989121159180,745898989121159208,745898989108576260,745898989121159172,745898989121159194
745898989121159188,745898989125353477,745898989116964875,745898989121159198,745898989108576264,745898989121159178
747357984694509568,747357984904224768,747357984749035520,747357984807755776,747357984849698816,747357984644177920,747357984883253248,747357984715481088,747357984770007040,747357984828727296
747357984707092480,747357984795172864,747357984853893120,747357984895836160,747357984665149440,747357984736452608,747357984820338688,747357984866476032,747357984509960192,747357984916807680
745898989121159196
745898989121159200
762832940033040386
715342356385325082

要多得多，出现group_concat把数据给截断了，什么原因呢？

解决问题

由于group_concat有长度限制,默认1024个字符，
所以解决的办法也就是将group_concat有长度限制设置足够大即可。
1）临时解决方案：
在命令行运行

SET SESSION group_concat_max_len = 18446744073709551615;
set global max_allowed_packet = 2*1024*1024*10;

2）永久解决方案：
首先找到mysql的配置文件，设置如下配置

max_allowed_packet = 20M
group_concat_max_len = 18446744073709551615