转载自:http://blog.sina.com.cn/s/blog_4e808acf01009qna.html (作者:miky)
GROUP_CONCAT()是MySQL数据库提供的一个函数,通常跟GROUP BY一起用,具体可参考MySQL官方文挡:http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_group-concat。
语法:
GROUP_CONCAT([DISTINCT] expr [,expr ...] [ORDER BY {unsigned_integer | col_name | expr} [ASC | DESC] [,col_name ...]] [SEPARATOR str_val])
1.例如:
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 |
+------------+---------+
这 就不需要用php循环了
$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']);
2.当然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:
SELECT student_id, GROUP_CONCAT(courses_id SEPARATOR '|||') AS courses FROM student_courses WHERE student_id=2 GROUP BY student_id;a.int字段的连接陷阱
+------------+---------+
| student_id | courses |
+------------+---------+
| 2 | 3|||4|||5 |
+------------+---------+
3.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:
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;
+------------+---------+
| student_id | courses |
+------------+---------+
| 2 | 5,4,3 |
+------------+---------+
4.需要注意的: