How to use Mysql's GROUP_CONCAT() function_PHP tutorial
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
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
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.
PHP code
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If you use the GROUP_CONCAT() function and the GROUP BY statement, it is very simple, as shown below:
SQL code
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This way the processing in php is simple:
PHP code
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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");
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
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
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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