MySQL truncates concatenated result of a GROUP_CONCAT function

As Jocelyn mentioned, the size of a GROUP_CONCAT() result is bounded by group_concat_max_len, however there is an additional interaction with ORDER BY that results in a further truncation to 1/3 of group_concat_max_len. For an example, see this related answer.

The default value for group_concat_max_len is 1024, and 1024 / 3 = 341 probably explains why the type of concat_products shows up as varchar(341) in the original example. If you were to remove the GROUP BY productId clause, concat_products should show up as varchar(1024).

I have not found this interaction between GROUP_CONCAT() and ORDER BY mentioned in the MySQL Manual, but it affects at least MySQL Server 5.1.

As I already wrote in an earlier comment, the MySQL manual says:

Values in VARCHAR columns are variable-length strings. The length can be specified as a value from 0 to 65,535.

So the problem is not with the data type of the field.

The MySQL manual also says:

The result is truncated to the maximum length that is given by the group_concat_max_len system variable, which has a default value of 1024. The value can be set higher, although the effective maximum length of the return value is constrained by the value of max_allowed_packet. The syntax to change the value of group_concat_max_len at runtime is as follows, where val is an unsigned integer: SET [GLOBAL | SESSION] group_concat_max_len = val;

Your options for changing the value of group_concat_max_len are:

1. changing the value at MySQL startup by appending this to the command:
   --group_concat_max_len=your_value_here
2. adding this line in your MySQL configuration file (mysql.ini): group_concat_max_len=your_value_here
3. running this command after MySQL startup:
   SET GLOBAL group_concat_max_len=your_value_here;
4. running this command after opening a MySQL connection:
   SET SESSION group_concat_max_len=your_value_here;

Documentation: SET, Server System Variables: group_concat_max_len