Querying a MySQL database to show a selected row and ALL rows - php

Question

I have a problem at the moment where I'm trying to get all rows plus a selected row from my tables.
So I have three tables - users, tasks, and a joining table called tasks_users
The latter table just contains two columns - task_id and user_id, for the purpose of normalization.
I have a form to edit a task. The problem arises when I want to retrieve ALL users (so the task can be reassigned) and I also want to show the CURRENT user. Given my current queries I can only get all the users, or just the single related user.
Am I overlooking something obvious, or do I need a more complex query? Or perhaps a couple of queries?
Further clarification:
Imagine you have an "edit task" form and on it you have the user currently assigned. I want to also show all other users, so that I may reassign. Given that I have three tables I'm struggling to construct a query that returns what I want. Currently I can only return ALL user OR the single assigned user.

Answer 1

It seems it's not about the current user, but the task assignee. No matter who assigns the task, he should not be able to assign it to the current assignee.
If only one user can be assigned to a particular task, you can remove task_users table, because simply adding column user_id in table tasks will do the same job.
Let's say you have in table users
id | username |
---------------------
1 | A |
2 | B |
3 | C |
4 | X |
5 | Y |
And in table tasks
id | title | user_id |
-----------------------------------
1 | Bla | 3 |
2 | Asd | NULL | // if this is even possible
If you want to retrieve who you can assign to task_id = 1 (Bla), you can simply do:
$currentTaskID = 1; // or however you retrieve them, it's just for the explanation purpose
SELECT id, username FROM users WHERE id != (SELECT user_id FROM tasks WHERE id = $currentTaskID);
The output will be:
id | username |
---------------------
1 | A |
2 | B |
4 | X |
5 | Y |
If a task can be assigned to more than one user, and you will keep task_users table, we can imagine you have there:
task_id | user_id |
-------------------------
1 | 3 |
2 | NULL |
1 | 5 |
You can query this way:
$currentTaskID = 1; // or however you retrieve them, it's just for the explanation purpose
SELECT id, username FROM users WHERE id NOT IN (SELECT user_id FROM task_users WHERE task_id = $currentTaskID);
So the output will be:
id | username |
---------------------
1 | A |
2 | B |
4 | X |
If you permit NULL values in assignee, so a task is not assigned to anyone, the SELECT will return all users, because of WHERE id != NULL when you have no NULL id's in users
For the purpose of showing the current user somewhere, but not available for assigning, you should separate the queries in functions/methods.
Saying you have:
function getAvailableAssignees($task_id) {
$sql = "SELECT id, username FROM users WHERE id != (SELECT user_id FROM tasks WHERE id = $task_id);";
$this->query($sql);
return $this->result();
}
and
function getCurrentAssignee($task_id) {
$sql = "SELECT user_id FROM tasks where id = $task_id;";
$this->query($sql);
return $this->result();
}
So once you need to show the available assignees, you call the respective method, so do, when you need to show the current assignee.

Answer 2

I think you might be looking for a UNION ALL
SELECT username, status from table
UNION ALL
SELECT username, status from table where status = 'current' LIMIT 1
Make sure that amount of columns from first query matches the amount of columns from second query. Don't do SELECT * FROM in one query and SELECT username from in another.

Answer 3

I think you should do 2 queries, not 1.. It doesn't seem like the users table should be in the same data selection as the tasks and users selection.
If you insist on combining the results you can do use UNION DISTINCT to combine the results of 2 queries.
THe first query decides the columns to be returned so you should set it to be the joined tables since those are more columns. The users should also fit in the columns of the joined select since it already contains them.

Answer 4

Thank you for all the replies. I have however managed to fix the issue using a nested foreach loop:
<ul>
<?php foreach($stages as $stage):?>
<li>
<label>
<?php echo $stage['stage_id']; ?>
<input type="checkbox" name="stages[]" value="<?php echo $stage['stage_id']; ?>"
<?php
// loop through the assigned stages. if it matches the stage ID then echo checked
foreach ($assigned_stages as $assigned_stage)
{
if ($stage['stage_id'] === $assigned_stage['stage_id'])
{
echo 'checked';
}
}
?>
/>
<?php echo $stage['name']; ?>
</label>
</li>
<?php endforeach; ?>
</ul>
This is a very common case and I'm still unsure if this is the best approach, however it works without issue and there isn't a great deal of code involved, so for the time being I'm happy.