Mysql的GROUP_CONCAT()函数使用方法

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：

SQL代码

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;