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] [code]expr

[,

expr

...]
[ORDER BY {

unsigned_integer

|

col_name

|

expr

}
[ASC | DESC] [,

col_name

...]]
[SEPARATOR

str_val

])
[/code]

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

　　

Or:

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

　　

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 ''

.

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 = [code]val

;
[/code]
The return value is a nonbinary or binary string, depending on whether the arguments are nonbinary or binary strings. The result type is

TEXT

or

BLOB

unless

group_concat_max_len

is less than or equal to 512, in which case the result type is

VARCHAR

or

VARBINARY

. (Prior to MySQL 5.0.19,

GROUP_CONCAT()

returned

TEXT

or

BLOB

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