group_ Concat learning and configuration

Catalog

1、 background

2、 Set up

1、 background

          In the development process , On the data Group By At the same time , If you want to assemble multiple fields of the same type of data into a field separated by a special string, it returns , You can use MySQL Group splicing function group_concat.

2、 Set up

        group_concat The length of strings that can be spliced is limited to MySQL Server configuration , Default to allow splicing 1024 Byte length string , You can query through the following instructions :

show variables  like 'group_concat_max_len';

Execution results ：

         Be careful ： When the length to be spliced exceeds 1024, The database will be truncated automatically , In the production environment , There will be data loss . It can be solved in two ways .

        Temporary settings ： Disadvantages: when the database service is restarted , The following settings will fail

set session group_concat_max_len =  Specify the length of the change ;

        Permanent settings ： In the database my.ini Add the following configuration to the configuration file

group_concat_max_len =  Specify the length of the change 

        in addition , It can also be configured as follows , To configure group_concat The maximum of

group_concat_max_len = -1

          Be careful ： stay mysql Configuration file with configuration changes , The database service needs to be restarted .

3、 Use cases

        1） Table structure ：

CREATE TABLE `stu`  (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,
  `class` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,
  `sorce` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 321 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;

        2）SQL

--  Yes class Grouping , take id and name Concatenate a comma into a field 
select GROUP_CONCAT(DISTINCT id,',',name order by sorce desc separator '|') as msg
from stu
group by class;