from http://www.sofee.cn/blog/2007/07/16/90/

GROUP_CONCAT()是MySQL数据库提供的一个函数，通常跟GROUP BY一起用,具体可参考MySQL官方文挡:http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_group-concat。
先来看一下这个函数的语法：
GROUP_CONCAT([DISTINCT] expr [,expr ...]
[ORDER BY {unsigned_integer | col_name | expr}
[ASC | DESC] [,col_name ...]]
[SEPARATOR str_val])
下面就演示一下这个函数所带来的方便之处，首先建立一个学生选课表student_courses，并填充一些测试数据。
CREATE TABLE student_courses (
student_id INT UNSIGNED NOT NULL,
courses_id INT UNSIGNED NOT NULL,
KEY(student_id)
);
INSERT INTO student_courses VALUES (1, 1), (1, 2), (2, 3), (2, 4), (2, 5);
若要查找学生ID为2所选的课程，则使用下面这条SQL：
mysql> SELECT student_id, courses_id FROM student_courses WHERE student_id=2;
+------------+------------+
| student_id | courses_id |
+------------+------------+
| 2 | 3 |
| 2 | 4 |
| 2 | 5 |
+------------+------------+
3 rows in set (0.00 sec)
输出结果有3条记录，说明学生ID为2的学生选了3、4、5这3门课程。
放在PHP里，必须用一个循环才能取到这3条记录，如下所示：
query("SELECT student_id, courses_id FROM student_courses WHERE student_id=2") as $row) {
$result[] = $row['courses_id'];
}
...
?>
而如果采用GROUP_CONCAT()函数和GROUP BY语句就显得非常简单了，如下所示：
mysql> SELECT student_id, GROUP_CONCAT(courses_id) AS courses FROM student_courses WHERE student_id=2 GROUP BY student_id;
+------------+---------+
| student_id | courses |
+------------+---------+
| 2 | 3,4,5 |
+------------+---------+
1 row in set (0.00 sec)
看见没，在PHP里就不用循环了，如下所示：
query("SELECT student_id, GROUP_CONCAT(courses_id) AS courses FROM student_courses WHERE student_id=2 GROUP BY student_id");
$result = explode(',', $row['courses']);
...
?>
当然分隔符还可以自定义，默认是以“,”作为分隔符，若要改为“|||”，则使用SEPARATOR来指定，例如：
mysql> SELECT student_id, GROUP_CONCAT(courses_id SEPARATOR '|||') AS courses FROM student_courses WHERE student_id=2 GROUP BY student_id;
+------------+-----------+
| student_id | courses |
+------------+-----------+
| 2 | 3|||4|||5 |
+------------+-----------+
1 row in set (0.00 sec)
除此之外，还可以对这个组的值来进行排序再连接成字符串，例如按courses_id降序来排：
mysql> SELECT student_id, GROUP_CONCAT(courses_id ORDER BY courses_id DESC) AS courses FROM student_courses WHERE student_id=2 GROUP BY student_id;
+------------+---------+
| student_id | courses |
+------------+---------+
| 2 | 5,4,3 |
+------------+---------+
1 row in set (0.02 sec)