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Mysql system variable group_concat_max_len
(Publish Date: 2009-10-4 1:53pm, Total Visits: 9980, Today: 2, This Week: 26, This Month: 102)

The maximum allowed result length in bytes for the GROUP_CONCAT() function. The default is 1024.

mysql> show variables like 'group%';
+----------------------+-------+
| Variable_name        | Value |
+----------------------+-------+
| group_concat_max_len | 1024  |
+----------------------+-------+

You change the value of group_concat_max_len. Either in your config file or setting the session variable, e.g. "SET @@group_concat_max_len = 9999999;'