MySQL syntax for UPDATE + CASE WHEN + EXISTS + SELECT MAX - mysql

Question

EDIT: Working now, solution in the bottom of this post.
I have tried to create this query for hours now, without success:
UPDATE tasks
SET `Order`=
(
CASE WHEN
(
SELECT EXISTS
(
SELECT 1
FROM user_job_tasks ujt
WHERE ujt.JobID = :jobID AND ujt.TaskID = LAST_INSERT_ID()
)
)
THEN
(
SELECT `order` FROM
(
SELECT MAX(t.`Order`)+1 AS `Order`
FROM user_job_tasks ujt
LEFT OUTER JOIN tasks t ON ujt.TaskID = t.ID
WHERE ujt.JobID = :jobID
) AS temp
)
ELSE
(
1
)
END
)
WHERE ID = LAST_INSERT_ID()
Now, the point of this is to update tasks table's column Order where ID equals something (LAST_INSERT_ID() here).
If any records exist in user_job_tasks where jobID and taskID match the values, I want to set Order as the highest value + 1 of Order found in user_job_tasks where jobID matches the value.
If not, Order will be set 1.
I am only getting an error General error: 1093 You can't specify target table 'tasks' for update in FROM clause. I fail to find anything wrong in the query syntax.
Sorry for confusing explanation, perhaps I should just go to sleep.
Working solution:
UPDATE tasks
SET `Order`=
(
COALESCE
(
(
SELECT `Order` FROM
(
SELECT MAX(t.`Order`)+1 AS `Order`
FROM user_job_tasks ujt
LEFT OUTER JOIN tasks t ON ujt.TaskID = t.ID
WHERE ujt.JobID = :jobID
) AS temp
),
1
)
)
WHERE ID = LAST_INSERT_ID()

Answer 1

I am not 100% sure this does what you want, but I think it does:
UPDATE tasks
SET `Order`= coalesce((SELECT MAX(t.`Order`) + 1 AS `Order`
FROM user_job_tasks ujt JOIN
tasks t
ON ujt.TaskID = t.ID
WHERE ujt.JobID = :jobID
), 1)
WHERE ID = LAST_INSERT_ID();
This just looks for the maximum id (+ 1) for your records and then uses 1 if this doesn't exist.
EDIT:
MySQL doesn't like using the table being updated in a from clause. So the above is correctly stated using a second subquery:
UPDATE tasks
SET `Order`= coalesce((SELECT `Order`
FROM (SELECT MAX(t.`Order`) + 1 AS `Order`
FROM user_job_tasks ujt JOIN
tasks t
ON ujt.TaskID = t.ID
WHERE ujt.JobID = :jobID
)
), 1)
WHERE ID = LAST_INSERT_ID();
This limitation would also apply to your query.