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;