SQL Query - Not in a set of already in-use items - mysql

Question

I am trying to select jobs that are not currently assigned to a user.
Users table: id | name
Jobs: id | name
Assigned: id | user_id | job_id | date_assigned
I want to select all the jobs that are not currently taken. Example:
Users:
id | name
--------------
1 | Chris
2 | Steve
Jobs
id | name
---------------
1 | Sweep
2 | Skids
3 | Mop
Assigned
id | user_id | job_id | date_assigned
-------------------------------------------------
1 | 1 | 1 | 2012-01-01
2 | 1 | 2 | 2012-01-02
3 | 2 | 3 | 2012-01-05
No two people can be assigned the same job. So the query would return
[1, Sweep]
Since no one is working on it since Chris got moved to Skids a day later.
So far:
SELECT
*
FROM
jobs
WHERE
id
NOT IN
(
SELECT
DISTINCT(job_id)
FROM
assigned
ORDER BY
date_assigned
DESC
)
However, this query returns NULL on the same data set. Not addressing that the sweep job is now open because it is not currently being worked on.

Answer 1

SELECT a.*
FROM jobs a
LEFT JOIN
(
SELECT a.job_id
FROM assigned a
INNER JOIN
(
SELECT MAX(id) AS maxid
FROM assigned
GROUP BY user_id
) b ON a.id = b.maxid
) b ON a.id = b.job_id
WHERE b.job_id IS NULL
This gets the most recent job per user. Once we have a list of those jobs, we select all jobs that aren't on that list.

Answer 2

You can try this variant:
select * from jobs
where id not in (
select job_id from (
select user_id, job_id, max(date_assigned)
from assigned
group by user_id, job_id));

Answer 3

I think you might want:
SELECT *
FROM jobs
WHERE id NOT IN (SELECT job_id
from assigned
where user_id is not null
)
This assumes that re-assigning someone changes the user id on the original assignment. Does this happen? By the way, I also simplified the subquery.

Answer 4

First you need to be looking at a list of only current job assignments. Ordering isn't enough. The way you have it set up, you need a distinct subset of job assignments from Assigned that are the most recent assignments.
So you want a grouping subquery something like
select job_id, user_id, max(date_assigned) last_assigned from assigned group by job_id, user_id
Put it all together and you get
select id, name from jobs
where id not in (
select job_id as id from (
select job_id, user_id, max(date_assigned) last_assigned from assigned
group by job_id, user_id
)
)
As an extra feature, you could pass up the value of "last_assigned" and it would tell you how long a job has been idle for.