How to use Mysql's GROUP_CONCAT() function_PHP tutorial-PHP Tutorial-php.cn

How to use Mysql's GROUP_CONCAT() function_PHP tutorial

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Release： 2016-07-21 15:53:14

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Syntax:

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

Let’s demonstrate this function. First, create a student course selection table student_courses and fill in some test data.

SQL code

Copy code The code is as follows:

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); 

To find the course selected by student ID 2, use the following SQL:

SQL code

Copy code The code is as follows:

mysql> SELECT student_id, courses_id FROM student_courses WHERE student_id=2; 
+- -----------+----------------+ 
| student_id | courses_id | 
+------------+ ------------+                                                                              
+--------- ---+----------------+ 
3 rows IN SET (0.00 sec) 

 

The output result has 3 records, indicating the student ID Three courses, 3, 4, and 5, were selected for student 2. 

If placed in PHP, a loop must be used to get these 3 records, as shown below:

PHP code

Copy the code

The code is as follows: foreach ($pdo->query("SELECT student_id, courses_id FROM student_courses WHERE student_id=2") as $row) {

$result[] = $row['courses_id'];

} 

If you use the GROUP_CONCAT() function and the GROUP BY statement, it is very simple, as shown below:

SQL code

Copy code

The code is as follows: mysql> SELECT student_id, GROUP_CONCAT( courses_id) AS courses FROM student_courses WHERE student_id=2 GROUP BY student_id; 🎜>+------------+---------+ ----+---------+

1 row IN SET (0.00 sec)

 
This way the processing in php is simple: 

PHP code

Copy code

The code is as follows:

$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']); The

separator can also be customized. The default is "," as the separator. If you want to change it to "|||", use SEPARATOR to specify it, for example:

SQL code

Copy code The code is as follows:

SELECT student_id, GROUP_CONCAT(courses_id SEPARATOR '|||') AS courses FROM student_courses WHERE student_id=2 GROUP BY student_id; 

In addition, you can also sort the values of this group and then concatenate them into strings, for example, sort by courses_id in descending order:

SQL code

Copy code The code is as follows:

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;