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;
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