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mysql GROUP
作者:mmseoamin日期:2023-12-11

文章目录

    • 概要
    • 使用技巧
      • 1. 建表、插入数据
      • 2.以id分组,把age字段的值拼成一行,逗号分隔(默认)
      • 3.以id分组,把age字段的值拼成 一行,分号分隔
      • 4.以id分组,把去冗余的age字段的值打印在一行
      • 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
      • 发现问题
      • 解决问题

        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
        

        mysql GROUP,在这里插入图片描述,第1张

        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;
        

        mysql GROUP,在这里插入图片描述,第2张

        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;
        

        mysql GROUP,在这里插入图片描述,第3张

        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;
        

        mysql GROUP,在这里插入图片描述,第4张

        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; 
        

        mysql GROUP,在这里插入图片描述,第5张

        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