Deep understanding of Group_concat function in Mysql _mysql

This paper introduces the use of Group_concat functions in MySQL, such as select Group_concat (name).
Group_concat function in MySQL
The complete syntax is as follows:
Group_concat ([DISTINCT] The field to connect [order by Asc/desc sort field] [Separator ' delimiter]]
Basic Query
Mysql> select * from AA;
+------+------+
| id| name |
+------+------+
|1 | 10|
|1 | 20|
|1 | 20|
| 20|
|3 | 200 |
|3 | 500 |
+------+------+
6 rows in Set (0.00 sec)
Grouped by ID, print the value of the Name field on one line, separated by commas (default)
Mysql> Select Id,group_concat (name) from AA Group by ID;
+------+--------------------+
| id| Group_concat (name) |
+------+--------------------+
|1 | 10,20,20|
| 20 |
|3 | 200,500|
+------+--------------------+
3 Rows in Set (0.00 sec)
Grouped by ID, print the value of the Name field on one line, separated by semicolons
Mysql> Select ID,GROUP_CONCAT (name separator '; ') from the AA group by ID;
+------+----------------------------------+
| id| Group_concat (name separator '; ') |
+------+----------------------------------+
|1 | 10;20;20 |
| 20|
|3 | 200;500 |
+------+----------------------------------+
3 Rows in Set (0.00 sec)
Grouped by ID, the value of the Name field to be redundant is printed on one line,
Comma separated
Mysql> Select Id,group_concat (distinct name) from the AA group by ID;
+------+-----------------------------+
| id| Group_concat (distinct name) |
+------+-----------------------------+
|1 | 10,20|
| 20 |
|3 | 200,500 |
+------+-----------------------------+
3 Rows in Set (0.00 sec)
Group by ID, print the value of the Name field on one line, comma separated, and then name in reverse order
Mysql> Select Id,group_concat (name order BY name Desc) from the AA group by ID;
+------+---------------------------------------+
| id| Group_concat (name order BY name Desc) |
+------+---------------------------------------+
|1 | 20,20,10 |
| 20|
|3 | 500,200|
+------+---------------------------------------+
3 Rows in Set (0.00 sec)
Using the Group_concat_max_len system variable, you can set the maximum allowable length. The syntax for doing this in a program is as follows, where Val is an unsigned integer:
SET [Session | GLOBAL] Group_concat_max_len = val;
If the maximum length has been set, the result is the maximum length.
Increase the environment variable Group_concat_max_len. The default is 1024. I set the session-level environment variable to 2048 (not enough to increase it). Solve the problem