如何使用php loop在mysql中将数据更新为null？

Question

I have a table named users with a column called user_subs. It looks like this.

In user_subs I have stored the specific users session username. Lets say this specific users name is James.

Now how would I loop through a specific user_subs looking for "James" and remove him from that specific user_subs without removing all the other names.

This is what I have so far and the only problem is, its deleting all the usernames in user_subs instead of just "James".

if(isset($_GET['p_id'])) {
  $the_post_id = $_GET['p_id'];
  $the_post_author = $_GET['author'];
}

if(isset($_POST['delete_sub'])) {
  $username = $_SESSION['username'];

  $query = "SELECT user_subs FROM users WHERE username = '{$username}' ";
  $select_users_by_id = mysqli_query($connection, $query); 

  while ($row = mysqli_fetch_array($select_users_by_id)) {
    $user_subs = explode(',', $row['user_subs']);

    foreach($user_subs as $out) {

      $query = "UPDATE users SET user_subs = null WHERE username = '{$the_post_author}' ";

       $unsubscribe_user = mysqli_query($connection, $query);

       echo "Unsubscribed";
    }
  }
}

THIS IS JUST IN TEST, PREPARED STATEMENTS WILL BE USED BEFORE GOING LIVE

Thank you for your time.

Answer 1

I second the other user's comment about moving this column to a different table. In the meanwhile, if you want to achieve what you are asking for, you can try removing the user name from the column value and update it with the remaining text.

if(isset($_POST['delete_sub'])) {
  $username = $_SESSION['username'];

  $query = "SELECT user_subs FROM users WHERE username = '{$username}' ";
  $select_users_by_id = mysqli_query($connection, $query); 

  while ($row = mysqli_fetch_array($select_users_by_id)) {
    $user_subs = str_replace($username . ',', '', $row['user_subs']);

$query = "UPDATE users SET user_subs = '{$user_subs}' WHERE username = '{$the_post_author}' ";

       $unsubscribe_user = mysqli_query($connection, $query);
       echo "Unsubscribed";

  }
}

OPTION-2

$user_subs = explode(',', $row['user_subs']);
$user_subs_new = [];
foreach($user_subs as $out) {
    if ($out !== $username) {
        $user_subs_new[] = $out;
    }
}
$user_subs = implode(',',user_subs_new);
  $query = "UPDATE users SET user_subs = '{$user_subs}' WHERE username = '{$username}' ";

   $unsubscribe_user = mysqli_query($connection, $query);

   echo "Unsubscribed";
}

Answer 2

Let's start over. Let's start here, in fact...

DROP TABLE IF EXISTS users;

CREATE TABLE users
(user_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY
,email VARCHAR(20) NOT NULL UNIQUE
);

DROP TABLE IF EXISTS user_subs;

CREATE TABLE user_subs
(user_id INT NOT NULL
, sub_id INT NOT NULL
, active TINYINT NOT NULL DEFAULT 1
, PRIMARY KEY(user_id,sub_id)
);

INSERT INTO users VALUES 
(1,'b.smyth634@gmail.com'),
(2,'james@gmail.com'),
(3,'f@gmail.com'),
(4,'sally@gmail.com'),
(5,'thomas@gmail.com');

INSERT INTO user_subs (user_id,sub_id) VALUES
(1,5),
(1,2),
(1,1),
(1,4),
(2,1),
(2,2),
(2,4);


SELECT * FROM users;
+---------+----------------------+
| user_id | email                |
+---------+----------------------+
|       1 | b.smyth634@gmail.com |
|       2 | james@gmail.com      |
|       3 | f@gmail.com          |
|       4 | sally@gmail.com      |
|       5 | thomas@gmail.com     |
+---------+----------------------+

SELECT * FROM user_subs;
+---------+--------+--------+
| user_id | sub_id | active |
+---------+--------+--------+
|       1 |      5 |      1 |
|       1 |      2 |      1 |
|       1 |      1 |      1 |
|       1 |      4 |      1 |
|       2 |      1 |      1 |
|       2 |      2 |      1 |
|       2 |      4 |      1 |
+---------+--------+--------+

SELECT u.*
     , GROUP_CONCAT(us.sub_id) subs 
  FROM users u 
  JOIN user_subs us 
    ON us.user_id = u.user_id 
 GROUP 
    BY u.user_id;
+---------+----------------------+---------+
| user_id | email                | subs    |
+---------+----------------------+---------+
|       1 | b.smyth634@gmail.com | 1,2,4,5 |
|       2 | james@gmail.com      | 1,2,4   |
+---------+----------------------+---------+

From here we have a choice. We can either DELETE subs we no longer wish to consider, or simply UPDATE them as 'inactive'.

Either way, we just need a DELETE or an UPDATE . So no SELECT needed. In fact a SELECT would, as I mentioned, be counterproductive - because a user may modify the data set in between the execution of the SELECT and the execution of the UPDATE/DELETE . This is known as a 'race condition'.