联接3个MySQL表时遇到问题，无法从两个表中检索数据（CodeIgniter）

Question

I have a database that has a structure like so (simplified for clarity):

user

• id
• name

task

• id
• name
• group

task_link

• user
• task

The query I'm attempting is, basically: "Get all tasks that are part of this group, along with all users that are assigned to that task" .

I've used JOIN queries in a few places on this project, but my understanding of them is basic, this unfortunately is a different scenario to the others. My attempt at a JOIN query seems to almost work, it looks like (CI Database class formatting, but hopefully it's clear what's happening regardless):

$this->db->select('task.name AS taskname, user.name AS username');
$this->db->from('task');
$this->db->from('task_link');
$this->db->where('task.group', $group);
$this->db->join('user', 'user.id = task_link.user');

Edit: Dumped as a proper query for clarity:

SELECT `user`.`name` AS username, `task`.`name` AS taskname FROM (`task`, `task_link`) JOIN `user` ON `user`.`id` = `task_link`.`user` WHERE `task`.`group` = '7'"

I've tried the above with almost every combination of task, task_link and user and end up with the same results each time. Main problems being:

• I get duplicate results, and an odd number of them - I can't quite work out what the grouping is. I seem to be getting 6 results for each task, which is currently the number of 'links'.
• The way that users are 'attached' to the task isn't right; I need the users returned as an array as part of the task (as each task may have multiple users linked to it); whereas I only seem to end up with one user in the result, and multiple results (but not a task for each user).

If it helps understand my model, the view would look something like:

foreach($tasks as $task){
  echo $task->taskname;
  foreach($task->username as $user){
    echo $user;
  }
}

I know that there are different JOIN clauses, and perhaps more relevant, grouping commands - but my brain is starting to hurt and I think I'm up against the edge of my mysql skills.

Is anyone able to lend a hand getting the above query to work as described? Please just let me know if I've omitted something useful!

Answer 1

I am guessing on how task_link and task are related but I think the basic SQL you want is:

SELECT user.name AS username, task.name AS taskname
FROM task
INNER JOIN task_link ON task.id = task_link.task
INNER JOIN user ON user.id = task_link.user
WHERE task.group = '7'"

Which I think translates to this:

$this->db->select('task.name AS taskname, user.name AS username');
$this->db->from('task');
$this->db->where('task.group', $group);
$this->db->join('task_link', 'task.id = task_link.task');
$this->db->join('user', 'user.id = task_link.user');