使用GROUP_CONCAT语法_MySQL

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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Java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：Java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Java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 Java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Java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 Java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Java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; bitsCN.com