GROUP_CONCAT(expr)

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 full syntax is as
follows:

GROUP_CONCAT([DISTINCT] expr [,expr ...]
[ORDER BY {unsigned_integer | col_name | expr}
[ASC | DESC] [,col_name ...]]
[SEPARATOR str_val])

Or:

In MySQL, you can get the concatenated values of expression combinations. To
eliminate duplicate values, use the class="literal">DISTINCT clause. To sort values in the result, use
the ORDER BY clause. To sort in reverse
order, add theDESC (descending) keyword to the
name of the column you are sorting by in the ORDER
BY clause. The default is ascending order; this may be specified
explicitly using the ASC keyword. The
default separator between values in a group is comma (“class="quote">,”). To specify a
separator explicitly, use class="literal">SEPARATOR followed by the string literal value that
should be inserted between group values. To eliminate the separator altogether,
specify SEPARATOR ‘‘.

The result is truncated to the maximum length that is given by the class="literal">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 class="literal">max_allowed_packet. The syntax to change the value
ofclass="literal">group_concat_max_len at runtime is as follows,
where val is an unsigned
integer:

SET [GLOBAL | SESSION] group_concat_max_len = val;

The return value is a nonbinary or binary string, depending on whether the
arguments are nonbinary or binary strings. The result type is class="literal">TEXT or class="literal">BLOB unless class="literal">group_concat_max_len is less than or equal to
512, in which case the result type is class="literal">VARCHAR or class="literal">VARBINARY. (Prior to MySQL 5.0.19, class="literal">GROUP_CONCAT() returned class="literal">TEXT or class="literal">BLOBclass="literal">group_concat_max_len greater than 512 only if
the query included an ORDER
BY clause.)

quoted from:

https://dev.mysql.com/doc/refman/5.5/en/group-by-functions.html#function_group-concat

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