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語法：

GROUP_CONCAT([DISTINCT] expr [,expr ...][ORDER BY {unsigned_integer | col_name | expr}[ASC | DESC] [,col_name ...]][SEPARATOR str_val])

下面演示一下這個函數(shù)，先建立一個學生選課表student_courses，并填充一些測試數(shù)據(jù)。

SQL代碼

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);    

若要查找學生ID為2所選的課程，則使用下面這條SQL：

SQL代碼

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)  

輸出結果有3條記錄，說明學生ID為2的學生選了3、4、5這3門課程。
放在PHP里，必須用一個循環(huán)才能取到這3條記錄，如下所示：



PHP代碼

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()函數(shù)和GROUP BY語句就顯得非常簡單了，如下所示：



SQL代碼

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)   

這樣php里處理就簡單了：



PHP代碼

$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']);     

分隔符還可以自定義，默認是以“,”作為分隔符，若要改為“|||”，則使用SEPARATOR來指定，例如：



SQL代碼

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，還可以對這個組的值來進行排序再連接成字符串，例如按courses_id降序來排：

SQL代碼

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;