Prioritize By Values Within Group - mysql

Question

In MySQL, say I have the following table (called workers):
| id | specialty | status | name
| :- | :-------- | :--------- | :--- |
| 1 | Bricks | Unemployed | Joe
| 2 | Bricks | Employed | Eric
| 3 | Bricks | Contracted | Bob
| 4 | Tiles | Employed | Dylan
| 5 | Tiles | Contracted | James
In my query, say I want to find who is a prospective person for a new job. Thus, I would want to first find who is Unemployed, if no one is Unemployed, then who is only Contracted, and if no one is Contracted then at least who is Employed.
This would be GROUP BY specialty. The only methods I could figure out are either complex sub-queries or sets of UNIONs (or both). I also tried GROUP_CONCAT however this didn't work (or I didn't do it right). Googling this has not yielded any results.
Another idea is to assign a value to each category, and then do a group-wise max/min sub-query. I piloted this and it works, however seems quite messy and definitely not normalized:
SELECT
`id`,
`name`,
`status`,
-- I haven't been able to figure out how to get rid of MIN from the actual select
-- statement except by wrapping this in another sub-query, which I'm not keen on
MIN(`priority`) AS `priority`
FROM workers
INNER JOIN (
SELECT 'Unemployed' AS `status`, 0 AS `priority` FROM dual UNION
SELECT 'Contracted' AS `status`, 1 AS `priority` FROM dual UNION
SELECT 'Employed' AS `status`, 2 AS `priority` FROM dual
) AS priorities USING (`status`)
GROUP BY `specialty`;
I am looking for a more standard, efficient, normalized or versatile method of doing this.
Update:
An additional method I could be to use a CASE expression in the SELECT clause of the statement. This would be if I were to normalize the status column, through a foreign-key relationship or other related table:
New table called statuses
| id | status |
| :- | :------------- |
| 1 | Employed |
| 2 | Contracted |
| 3 | Unemployed |
| 4 | Not contracted |
Diffs: 'Not Contracted' is a new status and my workers table now stores the foreign key to the new statuses table.
Then my SQL would be:
SELECT
`id`,
`name`,
statuses.status,
MIN(`priority`) AS `priority`
FROM workers
INNER JOIN (
SELECT
`id`,
`status`,
CASE
-- currently uses text in `status`,
-- could also explicitly use `id`
WHEN `status` IN ('Unemployed', 'Not Contracted') THEN 0
WHEN `status` = 'Contracted' THEN 1
WHEN `status` = 'Employed' THEN 2
ELSE 3
END AS `priority`
FROM statuses
) AS statuses ON workers.status = statuses.id
GROUP BY `specialty`;
Note: You might think - why not put the priority in the statuses table? The reason why I am not doing that is because the priority changes depending on the data needed / the purpose of the report being generated.
Potentially this is a cleaner solution (for the times that the related data to prioritize against is in another table). Again, I am looking for a more standard, efficient, normalized or versatile method of doing this. Also, if there is more of a way this could be configurable to user input / variables.

Answer 1

The difficulty here mainly arises because you don't have an ordinal column which ranks the various status in some order. Absent that, we can introduce one using a CASE expression, similar to what your second query is trying to do:
SELECT w1.*
FROM workers w1
INNER JOIN
(
SELECT
specialty,
MIN(CASE status WHEN 'Unemployed' THEN 1
WHEN 'Contracted' THEN 2
ELSE 3 END) AS status_rnk
FROM workers
GROUP BY specialty
) w2
ON w1.specialty = w2.specialty AND
w2.status_rnk = CASE w1.status WHEN 'Unemployed' THEN 1
WHEN 'Contracted' THEN 2
ELSE 3 END;