从数据库Codeigniter中加入的表格中选择一个列

Question

I am trying to select column name level_name from table levels , which contains level_id and level_name , for a user to know what is their level,

The table of users named as users and contain a level_id and user_id , but I get this error ->

Column 'level_id' in on clause is ambiguous

SELECT `level_name` 
 FROM `levels` 
   JOIN `users` ON `level_id` = `level_id` WHERE `user_id` = '9'

here it is the code in the model

public function level_ownprofile($user_id)
{
    $this->db->select('level_name');
    $this->db->from('levels');
    $this->db->join('users', 'level_id = level_id');
    $this->db->where('user_id', $user_id);
    $query = $this->db->get();
    return  $query;
}

thanks in advance :)

Answer 1

Select l.level_name 
FROM levels l
JOIN users u
ON u.level_id = l.level_id 
and u.user_id = '9'


public function level_ownprofile($user_id)
{
$this->db->select('level_name');
$this->db->from('levels');
$this->db->join('users', 'levels.level_id = users.level_id');
$this->db->where('user_id', $user_id);
$query = $this->db->get();
return  $query;
}

Answer 2

Change the query to

SELECT `level_name` 
 FROM `levels` l
   JOIN `users` u ON `u`.`level_id` = `l`.`level_id` 
 WHERE `user_id` = '9'

if you like the table name aliasing method, it is shorter and easier to read.

Or

SELECT `level_name` 
 FROM `levels` 
   JOIN `users` ON `users`.`level_id` = `levels`.`level_id` 
WHERE `user_id` = '9'

If you prefere to use the full table name everywhere.

Because both tables contain a column with the name level_id the query analyser need to know which one you are addressing.

In codeigniter try

public function level_ownprofile($user_id)
{
    $this->db->select('level_name');
    $this->db->from('levels l');
    $this->db->join('users u', 'u.level_id = l.level_id');
    $this->db->where('user_id', $user_id);
    $query = $this->db->get();
    return  $query;
}

Answer 3

public function level_ownprofile($user_id)
{
    $this->db->select('l.level_name');
    $this->db->from('levels as l');
    $this->db->join('users as u', 'l.level_id = u.level_id');
    $this->db->where('l.user_id', $user_id);
    $query = $this->db->get();
    return $query->results();
}