如何编写选择查询以在mysqli中按父ID获取所有子ID

Question

I am trying to get all child ids by parent id from a single table. Below is my table course :

Here is my code:

public function getLeftMenuMainCategoryInfoFromDb()
{
    $sqlQuery = "SELECT p.course_id AS parent_id ,p.course_name As parent_coursename ,c.course_id AS child_id ,c.course_name As child_coursename FROM course As p LEFT JOIN course As c ON c.course_parent_id = p.course_id WHERE p.course_parent_id = 0 ORDER BY p.course_id";
    $result = $this->selectQuery($sqlQuery);
    //print_r($result);
    return $result;

}

Answer 1

Try this Query SELECT GROUP_CONCAT(course_id),course_parent_id FROM courseGROUP BY course_parent_id

SELECT GROUP_CONCAT(course_id),course_parent_id FROM course WHERE course_parent_id =1 GROUP BY course_parent_id

IF this is not you want please elaborate your question.

Answer 2

I think are you looking for Parent ID should be GROUP BY

SELECT p.*,
       q.course_name
FROM course p
INNER JOIN
  (SELECT course_id,
          course_name
   FROM course
   WHERE course_parent_id = 0
   ORDER BY course_id) q ON p.course_parent_id = q.course_id
GROUP BY p.course_parent_id;