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;