GROUP_CONCAT(expr)

In MySQL, you can get the concatenated values of expression combinations. To eliminate duplicate values, use the DISTINCT clause. To sort values in the result, use the ORDER BY clause. To sort in reverse order, add the DESC (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 (,). To specify a separator explicitly, use SEPARATOR followed by the string literal value that should be inserted between group values. To eliminate the separator altogether, specify SEPARATOR ''.

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

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:

SELECT student_name,
         GROUP_CONCAT(test_score)
       FROM student
       GROUP BY student_name;

When setting the value for group_concat_max_len, consider the following:

SELECT student_name,
         GROUP_CONCAT(DISTINCT test_score
                      ORDER BY test_score DESC SEPARATOR ' ')
       FROM student
       GROUP BY student_name;

Estimate the maximum length required for GROUP_CONCAT() output and set the value accordingly.

SET [GLOBAL | SESSION] group_concat_max_len = val;