使用GROUP_CONCAT语法

语法：
GROUP_CONCAT([DISTINCT] expr [,expr ...][ORDER BY {unsigned_integer | col_name | expr}[ASC | DESC] [,col_name ...]][SEPARATOR str_val])
下面演示一下这个函数，先建立一个学生选课表student_courses，并填充一些测试数据。
SQL代码

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：

PHP代码

foreach ($pdo->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：

SQL代码

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)

这样php里处理就简单了：

PHP代码

$row = $pdo->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'
]);

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：

SQL代码

SELECT student_id, GROUP_CONCAT(courses_id SEPARATOR 
'|||'
) AS courses FROM student_courses WHERE student_id=
2
 GROUP BY student_id;

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：
SQL代码

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;