http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_group-concat
* GROUP_CONCAT()
[...]
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;
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Hartmut Holzgraefe, MySQL Regional Support Manager EMEA
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