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  MySQL function: group_concat () function
  Add Date : 2018-11-21      
This function will merge non-null column values ​​in accordance with the grouping condition and eventually returned. If a null value is returned is empty, the complete syntax is as follows:
GROUP_CONCAT ([DISTINCT] expr [, expr ...]
            [ORDER BY {unsigned_integer | col_name | expr}
                [ASC | DESC] [, col_name ...]]
            [SEPARATOR str_val])
Example Column 1:
    SUM (VALUE1),
    SUM (VALUE2),
    SUM (VALUE3),

Column 2:

    SUM (VALUE1),
    SUM (VALUE2),
    SUM (VALUE3),

The above column 1 and column 2 the difference is that the column 2 is the field REAS_1, REAS_2, REAS_3 field one by one as a complete merger.
The maximum length of the receive function group_concat_max_len environment variables defined, it defaults to 1024, of course, can be set higher. At the same time the effective maximum length is returned by the max_allowed_packet constraints.
Group_concat_max_len running is to modify the following syntax:
SET [GLOBAL | SESSION] group_concat_max_len = val;

The result returned is a binary or non-binary character, depending on the value of the function within the field itself. Usually when group_concat_max_len < = 512, in order to VARCHAR or VARBINARY, when the value is greater than the TEXT and BLOB.
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