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  MySQL group_con cat_max_Len
  Add Date : 2018-11-21      
  MySQL provides group_concat function can splice a field value to a string, such as select group_concat (user_name) from sys_user, the default separator is a comma, that "" if you need custom delimiter can be used SEPARATOR

Such as: select group_concat (user_name SEPARATOR '_') from sys_user

However, if the length of the byte string user_name splicing more than 1024 will be truncated.

The command "show variables like 'group_concat_max_len'" to see group_concat default length:

mysql> show variables like 'group_concat_max_len';
+ -------- + ------- +
| Variable_name | Value |
+ -------- + ------- +
| Group_concat_max_len | 1024 |
+ -------- + ------- +
1 row in set

Add disposed MySQL configuration file: group_concat_max_len = -1 (-1 maximum or according to actual demand set the length), you need to restart MySQL after the configuration service to view as follows:

mysql> show variables like 'group_concat_max_len';
+ ---------------------- + ------------ +
| Variable_name | Value |
+ ---------------------- + ------------ +
| Group_concat_max_len | 4294967295 |
+ ---------------------- + ------------ +
1 row in set

If this is a production environment, can not be allowed to restart the MySQL service, you can set the scope group_concat by statements such as:

 SET GLOBAL group_concat_max_len = -1;

 SET SESSION group_concat_max_len = -1;
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