Syntax
------ 
GROUP_CONCAT(expr)
 
Description
----------- 
This function returns a string result with the concatenated
non-NULL
values from a group. It returns NULL if there are no
non-NULL values.
 
The maximum returned length in bytes is determined by the
group_concat_max_len server system variable, which defaults
to 1M (>= MariaDB 10.2.4) or 1K (

URL: https://mariadb.com/kb/en/group_concat/